The news on Friday followed RSK's acquisition of consultancy firm Building Sciences earlier this month, making RSK the largest privately-owned environmental consultancy in the UK.
RSK Group chairman, Dr Alan Ryder, commented: "This is the second largest acquisition in RSK's history and represents a major statement of intent for us moving forward.
"We’ve strengthened our capabilities across the board and brought in services that we have never had before. STATS will be instrumental to our continued growth and success."
STATS, formerly known as St. Albans Testing Services, has been involved in a wide range of historic UK infrastructure projects since its inception in 1974.
It was initially focused on providing inspection services and as design materials consultants, however in recent years they have increasingly featured in the environmental sector, something they are keen to work on with RSK Group.
STATS services have been utilised on projects such as the Channel Tunnel Rail Link, during the rail and bus privatisation schemes in the 1990s, and by the Department for Transport, who commissioned them to provide a definitive technical guide to performing investigations on concrete bridges.
Speaking to NCE this week about the merger, STATS chairman, Adrian Marsh, explained: "There are various different reasons for RSK buying us. Fundamentally, we have very complimentary skills & complimentary client bases, so it made sense that we should pool our resources."
He added: "Over the years the procurement processes for clients have changed in favour of the larger practices. Although in the past we’ve been involved in some bigger projects, now we need to be part of a bigger organisation to compete."
Marsh is assured that both there are larger gains to be made by both parties. He said: "It's a genuine two-way process; we bring the more traditional practices they lack, such as the building acoustics work, and conversely they provide things such as the carbon management expertise which we were lacking."
RSK Group grew quickly during 2007 with acquisitions including Carter Ecological, ESA (ground remediation), and Structural Soils, helping them to cement their position in the market.