PEO, ASO and HRO name three ways of buying outside help with employment administration, and the differences are structural, not cosmetic. A PEO co-employs your staff and can sponsor benefits and, when IRS-certified, take on employment tax liability. An ASO, administrative services only, does the same operational work while you remain the sole employer and everything files under your EIN. HRO, HR outsourcing, is the broadest label: handing selected functions, from payroll to recruiting, to a provider, in whatever combination you contract for. The choice determines whose name is on the filings, who sponsors the benefits, and who answers when a deposit is missed.
The three models in one paragraph each
A PEO enters a co-employment relationship: it becomes an employer of record for administrative purposes, bundles payroll, benefits, compliance assistance and HR services, and prices per employee or as a share of payroll. The industry body NAPEO reports about 14% of US employers with 20 to 499 staff use one. An ASO delivers similar administration, payroll processing, benefits administration, HR support, without co-employment: you remain the only employer, plans are yours, filings are under your EIN. HRO is a scope label rather than a legal structure: a contract to run chosen HR functions, from a single process to the whole department, and it can sit on top of your own systems or the provider's.
Whose EIN, whose liability
The IRS's third-party arrangement guidance is the map. A payroll service provider or reporting agent, which is how ASO and most HRO payroll arrangements are treated, does not relieve the employer of its federal employment tax duties: returns run under your EIN and the liability for deposits and filings stays with you even when the provider caused the failure. A certified PEO is the exception the law carves out: for worksite employees under a CPEO contract, the CPEO is solely liable for paying employment taxes, filing returns and making deposits, filing in aggregate under its own EIN with a schedule allocating each client, with the relationship reported on Form 8973. A non-certified PEO sits in between: operationally similar, but without the statutory sole-liability shield, so the contract and the IRS default rules decide.
Benefits access: the practical difference
The clearest buyer-visible difference is benefits. A PEO sponsors plans across its whole client base, so a small employer's people enroll in large-group cover they could not buy alone; that pooling is the reason health insurance drives most PEO purchases. Under an ASO or HRO arrangement the plans are your own: the provider administers enrollment and billing, but the rates reflect your company's own size and risk. For a firm large enough to get strong rates directly, ASO preserves control and often costs less; for a small firm, the PEO's pooled benefits can outweigh its higher fee. This is also why leaving a PEO is a project: the plans stay with the sponsor, so an exit means re-procuring cover, not just changing vendors.
Choosing between the three
Start from what you want to own. If the goal is better benefits and less employment administration at small headcount, the PEO bundle fits, and certification determines whether tax liability genuinely moves. If you want to keep sole-employer status, your own plans and your own EIN while outsourcing the labour, ASO is the shape, priced lower because it transfers less. If the need is narrower, one function done well, a scoped HRO contract does it without restructuring anything. Pricing follows the transfer: PEOs cost the most per employee because they carry the most, ASOs less, single-function HRO least. Whatever the model, the service agreement controls what is actually owed, and nothing here is legal or tax advice.
Questions people ask about peo vs aso vs hro
Is an ASO just a PEO without co-employment?
Operationally close: similar administration, but you stay sole employer, plans are yours, filings run under your EIN and the IRS holds you responsible for employment taxes, as with any payroll service provider.
Does using any of the three move tax liability off my company?
Only a certified PEO, and only for worksite employees under the CPEO contract, where the CPEO is solely liable. ASO, HRO and non-certified PEO arrangements leave the employer's federal employment tax duties in place.
Which model is cheapest?
Scoped HRO for a single function, then ASO, then PEO, in rough order, because price tracks how much responsibility and bundled service transfers. The right comparison is total cost against what you would otherwise run in-house.
Can we move from a PEO to an ASO later?
Yes, and companies do as headcount grows and direct benefits rates improve. Plan the transition: PEO-sponsored plans stay behind, filings return to your EIN, and mid-year moves have wage-base implications worth reviewing with a tax adviser.