Pittsburgh-based System One announced private equity firm Thomas H. Lee Partners L.P. agreed to make a significant investment in the firm. The investment proceeds will be used to purchase the interests of middle-market private equity firm MidOcean Partners, System One’s current investment partner, which invested in the company in December 2012.

The agreement was completed Sept. 19, and the transaction is expected to close before the end of the year. Neither financial terms nor ownership structure were disclosed.

System One’s core specialty areas include energy, engineering, IT, commercial, clinical and scientific, and industrial staffing and services. Its integrated services also include a proprietary vendor management system, legal e-discovery, utility smart grid installation, and quality assurance/quality control managed services.

System One’s affiliated companies include Link2Consult, Compliance Discovery Solutions, Utility Partners of America, System One and Joulé. The company ranks No. 22 on Staffing Industry Analysts’ list of largest staffing firms in the US.

“We have very much enjoyed the backing by MidOcean, whose deep industry expertise and relationships have helped us significantly expand our service offerings and client base,” said System One CEO Troy Gregory. “As we enter the next phase of our growth, we are thrilled to announce our new partnership with THL, who as seasoned investors in our industry, share System One’s commitment to our customers, and have the experience and resources to support our efforts to expand our share in key markets.”

Thomas H. Lee Partners invests in growth-oriented businesses, headquartered principally in North America, across three broad sectors: consumer and healthcare, media and information services, and business and financial services. Since its founding in 1974, it has raised more than $20 billion of equity capital and invested in more than 130 businesses with an aggregate purchase price of more than $150 billion.