CHICAGO, July 30 -- For the 9,500 professionals at CBIZ Inc. CBZ, Wednesday's announcement arrived with the kind of finality a $55-per-share all-cash offer does not soften. Grant Thornton Advisors agreed to acquire the mid-market accounting and benefits firm in a deal valued at $5 billion, a 54 percent premium to CBIZ's 30-day volume-weighted average price, reorganizing the upper tier of American professional services below the four firms that have long dominated it.

The acquisition, disclosed July 29, gives Grant Thornton, a Chicago-based firm with roughly 9,000 US professionals and a global network of 76,000 staff across 156 markets, a substantial expansion into the tax, accounting, insurance, and benefits practices CBIZ has built for ...