Private equity firm MidOcean Partners made an equity investment in System One, one of the largest U.S. providers of engineering staffing, the companies announced today. The amount of investment was not released, but it was described as significant. The company also reported the System One management team remains a significant owner of the business.

System One ranks No. 6 on Staffing Industry Analysts’ list of largest engineering staffing firms. In addition, it ranks No. 42 in size among all U.S. staffing firms with 2011 staffing revenue of $300 million.

System One, based in Pittsburgh, provides technical outsourcing solutions to the energy, engineering, information technology, commercial and scientific sectors. Over the last three years, it has added complementary services including a proprietary vendor management system, legal staffing business and utility services business.

MidOcean Partners is based in New York.