Professional employer organization TriNet is acquiring Strategic Outsourcing Inc. (SOI), the fourth-largest PEO in the U.S., from Clarion Capital Partners LLC.

TriNet ranks No. 8 on Staffing Industry Analysts’ 2012 list of largest U.S. PEOs with estimated PEO revenue of $185 million. SOI had estimated PEO revenue of $311 million.

Charlotte, N.C.-based SOI will continue to operate under its own brand, and SOI’s senior management team will remain in place.

“The SOI service model will strengthen our ability to support a broader range of entrepreneurial companies that are putting people back to work,” said TriNet CEO Burton Goldfield.

TriNet expects to complete the acquisition Oct. 24.

TriNet is based in San Leandro, Calif.