The ASO PEO choice turns on a single structural difference: co-employment. A professional employer organization (PEO) co-employs your staff, files payroll taxes under its own employer identification number and sponsors benefit plans. An administrative services organization (ASO) does neither; it administers payroll and HR tasks while your company remains the sole employer of record, filing under its own EIN. Everything downstream, from liability to pricing to which benefits your employees see, follows from that split. This page lays out both models against the same questions so the comparison is like for like.
Co-employment is the whole difference
With a PEO, the vendor becomes the employer of record for administrative purposes, such as payroll and sponsoring workers' compensation or health insurance plans, and shares defined employer liabilities under a client service agreement. With an ASO, no co-employment arises: the vendor is a third-party administrator, your company keeps complete employment control, and every filing goes out under your company's EIN, which keeps the liability with you. Neither model touches who manages the work; in both cases hiring, firing and direction stay with the client. The question the choice answers is how much of the employer's administrative identity you want to hand over, not how much of the management.
Taxes, benefits and where liability sits
A PEO files payroll taxes under its own EIN, which is why the IRS built a certification program for the industry: CPEO status, created by the Tax Increase Prevention Act of 2014 under Internal Revenue Code section 7705, requires demonstrated financial responsibility and tax compliance, because clients are trusting the PEO to remit their withheld taxes. A PEO also sponsors insurance and retirement plans, pooling employees across its client base to negotiate terms a small employer cannot get alone. An ASO administers whatever plans your company sponsors itself: you keep your vendor relationships and customization, and you also keep the full compliance and funding risk on every one of them.
Pricing and fit
The pricing shapes differ with the structures. ASOs charge per-employee monthly rates, typically in the $50 to $250 range depending on the service depth. PEOs charge a percentage of payroll, commonly between 2% and 15%, or a flat per-employee fee, and the bundle usually includes workers' compensation and unemployment insurance administration. The fit guidance follows headcount and HR maturity: PEOs suit employers of roughly five to a hundred people without dedicated HR staff, while ASOs suit companies of about twenty-five or more that already have someone responsible for HR and want targeted administrative relief. As always, the client service agreement controls what is actually delivered; price the specific scope, not the acronym.
Questions people ask about aso peo
Does an ASO create co-employment?
No. The ASO is a third-party administrator; your company remains the sole employer of record, files taxes under its own EIN and keeps all employer liabilities. Co-employment is specifically the PEO structure.
Whose benefits do employees get under each model?
Under a PEO, employees join plans the PEO sponsors and prices across its whole client pool. Under an ASO, employees stay on plans your company sponsors; the ASO only administers them.
Which is cheaper, an ASO or a PEO?
ASOs generally cost less in fees, at per-employee monthly rates around $50 to $250, because they carry less risk and sponsor nothing. PEO fees, a percentage of payroll or a flat per-employee amount, buy the benefit pooling and tax administration that ASOs do not provide. Compare total cost including benefit premiums, not fees alone.
Can a company move from a PEO to an ASO?
Yes, and growing companies often do once they have in-house HR and want their own benefit plans. Time the exit carefully: leaving a PEO mid-year can reset payroll tax wage bases and disrupt employees' benefit continuity.