PEO and ASO are the two main shapes of HR outsourcing for companies that already employ their own staff, and the difference between them is structural. A PEO enters co-employment: it becomes the administrative employer, and payroll, benefits and employment tax mechanics run through its infrastructure. An ASO, administrative services only, does the same categories of work as a pure service provider: everything files under your company's own identification number and you remain the only employer. That one design choice drives the tax liability position, the benefits you can access and the price, which is why naming the model precisely matters more than comparing feature lists.
How the two models are built
Under a PEO agreement the workforce is co-employed: the client directs the work while the PEO acts as administrative employer, running payroll, remitting employment taxes, and sponsoring benefits and workers' compensation programs across its whole client base. NAPEO describes the offer as payroll, benefits, compliance assistance and other HR services, with the pooled scale giving small employers access to benefits they could not buy alone. An ASO strips out the co-employment: the provider administers payroll, filings and HR processes on your behalf, but every return runs under your EIN, benefits are your own plans, and no second employer appears anywhere. ASO fees are typically lower for the same administrative scope because no employment relationship or benefits sponsorship is being carried.
The liability question underneath
The IRS's third-party payer guidance is the cleanest lens: depending on the facts and the type of arrangement, an employer using a third party for employment tax functions may remain solely liable, become jointly liable, or be relieved of liability, and its general warning is that employers are ultimately responsible for withheld income tax and both portions of social security and Medicare taxes. An ASO sits at the transparent end, liability stays entirely with you, the provider is your agent in substance. A conventional PEO shifts mechanics without necessarily shifting ultimate liability. The named exception is the certified PEO: in certain situations, customers of a CPEO are relieved of liability for income tax withholding and social security and Medicare taxes, which is a statutory feature no ASO can offer.
Choosing between PEO and ASO
The PEO shape earns its higher fee when pooled benefits are the point: a small team getting large-group medical rates, integrated workers' compensation and a compliance function it could not staff. It costs flexibility, since benefits and processes arrive as the PEO designed them, and exits need planning because payroll history and benefits move with the relationship. The ASO shape fits companies that want their own plans and their own EIN on every filing, with the administrative load lifted; it keeps every obligation visibly yours, which some buyers count as a feature. In both cases the service agreement defines the real split of duties, so read it against the IRS's arrangement categories rather than against the brochure. Nothing on this page is legal or tax advice.
Questions people ask about peo and aso
What does ASO stand for and mean?
Administrative services only: a provider runs payroll, filings and HR administration on your behalf, but there is no co-employment, filings run under your own EIN, and you remain the sole employer and the liable party.
Is a PEO always safer than an ASO for tax compliance?
No. The IRS notes liability depends on the arrangement type, and that employers generally stay ultimately responsible. The specific liability relief the IRS names attaches to certified PEOs, in certain situations, not to the PEO label itself.
Why is a PEO usually more expensive than an ASO?
The PEO carries more: co-employment infrastructure, sponsored benefits and workers' compensation programs, and the compliance apparatus around them. An ASO sells administration alone, so the same task list prices lower.
Can I move from a PEO to an ASO later?
Yes, and companies do as they grow into running their own benefits. Plan the transition around plan years and payroll tax filing periods, since employment history, benefits and filings move from the PEO's structure back to your own.