Prohibited transactions are the sharpest edge in ERISA, because they are the rare place where the law does not ask whether your decision was reasonable. It asks whether a transaction happened between the plan and someone too close to it. If it did, and no exemption covers it, the transaction is prohibited — even if the plan came out ahead, even if nobody profited, even if it was an accident.
That is a hard concept for employers who are used to being judged on process. Most fiduciary duties are procedural: document your reasoning, act prudently, and you are defensible. The prohibited transaction rules are structural instead. They ask who was on the other side of the deal.
Below is what plan sponsors should actually watch inside their own plan — not exotic self-dealing schemes, but the ordinary operational situations where a company and its retirement plan quietly end up on both sides of a transaction.
Why the rule exists at all
ERISA and the parallel tax rules assume that when a plan deals with a person who has influence over it, arm’s-length pricing cannot be trusted. Rather than litigate fairness case by case, Congress banned whole categories of dealings between the plan and its related parties, then created narrow exemptions for the transactions plans genuinely need — paying reasonable fees for necessary services, for example.
Two consequences follow, and both matter to sponsors:
- Intent is irrelevant. There is no “we meant well” defense.
- Correction is affirmative. A prohibited transaction does not simply lapse. It has to be unwound, the plan made whole, and the event reported — including on the plan’s Form 5500, which the plan administrator signs under penalty of perjury.
Who counts as a related party
The statutory term is “party in interest” under ERISA (and “disqualified person” in the tax code). The definition is deliberately wide. In a typical employer-sponsored 401(k), it reaches:
- The employer sponsoring the plan, and companies related to it by ownership — which is why your controlled group analysis is a prohibited-transaction question too, not just a coverage question.
- Every fiduciary of the plan: the named fiduciary, trustees, committee members, and anyone exercising discretion in fact.
- Service providers to the plan — recordkeeper, TPA, advisor, auditor, custodian. Note that your vendors become parties in interest the moment they are engaged.
- Owners, officers, highly compensated employees of the employer, and certain family members.
- Employees of the plan and organizations in which the above hold substantial interests.
The practical takeaway: almost everyone your plan deals with day to day is a party in interest. That is normal. Ordinary plan operations run on exemptions, which is exactly why the exemption conditions have to be met and evidenced.
Where sponsors actually stumble
The classic textbook violations — the plan lending money to the owner, the plan buying the company’s building — are rare and usually deliberate. The violations that catch ordinary employers are operational. Five recur.
1. Late deposit of participant deferrals
This is by far the most common prohibited transaction in small and mid-sized plans, and most sponsors do not recognize it as one. Once money is withheld from a paycheck, it is a plan asset. Holding it in the company’s operating account past the point it could reasonably have been segregated is treated as the employer using plan assets — a loan from the plan to the employer. It is corrected through a documented process, not by simply depositing late and moving on. See our detailed walkthrough of how employers correct late deferral deposits.
2. Paying employer expenses out of plan assets
Plans may pay reasonable expenses of administering the plan. They may not pay the employer’s own business (“settlor”) costs — the decision to establish, amend, terminate, or redesign a plan, and the studies that support those decisions. Getting this line wrong turns a routine invoice into a plan-asset misuse. Fee allocation is best reviewed alongside your 408(b)(2) fee disclosures.
3. Unreasonable or undisclosed compensation
Hiring a service provider is exempt only if the services are necessary, the arrangement is reasonable, and no more than reasonable compensation is paid. If you cannot show what you are paying — including indirect revenue such as revenue sharing — you cannot show the exemption applies. That is why the fee review is a prohibited-transaction control, not a cost-cutting exercise.
4. Participant loans that fall outside plan terms
A loan from the plan to a participant is a transaction with a party in interest. It is exempt only if it is available on a reasonably equivalent basis, adequately secured, made per specific plan provisions, and bears a reasonable rate. A loan that ignores the plan’s written limits, or a loan repayment stream that quietly stops, can lose the exemption. We cover the mechanics in plan loan defaults and deemed distributions.
5. Self-interested selection of the plan’s own vendors
A fiduciary may not use plan authority to benefit themselves or a party they are connected to. In closely held businesses, this shows up as the owner steering the plan to a relative’s advisory practice, or an executive selecting a vendor that also serves the company on favorable terms. The selection may be defensible — but only if the conflicted person is out of the decision and the file shows it.
Employer stock and real property
Plans holding qualifying employer securities or employer real property live under their own exemption with strict conditions, including limits for defined contribution and defined benefit plans and a requirement that the acquisition be for adequate consideration. If your plan holds — or is being asked to hold — anything the employer issued or owns, that is not a decision to make internally. Get ERISA counsel and an independent valuation before the transaction, not after.
An eight-step self-check for sponsors
- Maintain a current party-in-interest list. Owners, officers, fiduciaries, vendors, affiliated entities, and known family relationships. Refresh it annually and after any ownership change.
- Reconcile deferral withholding to deposit dates every payroll cycle, and investigate every outlier in writing.
- Classify every plan invoice as a plan expense or a settlor expense before it is paid, and keep the rationale.
- Total all compensation flowing to each provider — direct and indirect — and benchmark it on a documented cadence.
- Audit outstanding loans against the plan document’s written limits and the actual payroll repayment records.
- Require conflict disclosures from every committee member, and recuse them from decisions where they have an interest.
- Confirm no plan asset touches the employer — no plan funds in company accounts, no plan property used by the business, no informal borrowing.
- Answer the Form 5500 party-in-interest questions from evidence, not from memory, before the plan administrator signs.
What you can delegate — and what you cannot
You can delegate the monitoring: the deposit reconciliation, the fee tracking, the loan audits, the document-versus-operation checks, and the preparation of the annual report. A capable 3(16) plan administrator should be running those controls continuously rather than discovering problems at audit time.
You cannot delegate what only you know. Nobody outside your company can list your affiliates, your owners’ family relationships, or which vendor relationships overlap with the business. And you cannot delegate the decision to enter into a transaction with a related party — that is a settlor or fiduciary act belonging to the employer. Understanding which fiduciary role covers what is the starting point for drawing that line honestly.
Where Admin316 stands
Most providers will monitor around this problem and disclaim responsibility for it. Admin316 accepts appointment as the ERISA 402(a) named fiduciary — the role most providers decline — and takes on the administrative decision-making that comes with it, alongside full 3(16) administration. That means the party-in-interest controls above are run as a standing process, with evidence, rather than reconstructed the week your auditor asks.
If you are not certain your plan could produce that evidence today, that uncertainty is the finding. Book a plan review with Admin316 and we will walk your plan’s related-party exposure with you.
