WPP broke in this month on the long-running talks between the six-year-old agency and Havas following the recent closure of its own London agency United.
WPP announced the news to the London Stock Exchange this morning without disclosing the sum paid, although judging by CHI's £18.5m revenue figure for the year ended June 30 2006, it can be estimated in the region of £15m-£20m.
The fine print of the deal will be hammered out over the next few weeks. It is not known whether the offer is all in cash or also includes WPP shares, and neither side would be drawn.
CHI will retain its independent positioning, with agency founders Simon Clemmow, Johnny Hornby and Charles Inge set for a large payout in return for part of their equity.
In addition, CHI will increase its 32% stake in direct marketing agency Hall Moore CHI to 100% and amalgamate its business. The partners Simon Hall and Warren Moore are to stay with the company.
According to the agency's finance director Peter Walker, there was no difference in the valuation WPP and Havas put on CHI, and the attraction for agency was the ability to access WPP's media buying capabilities and strength in the US.
Hornby said: "We can now fully support our clients both internationally and by becoming a new model full service agency. In so doing we have secured the long-term independence and security of CHI for the benefit of everyone in the business."
Buying into CHI will allow Sir Martin Sorrell, the WPP chief executive, to repair some of the damage done to its creative offering by United's collapse. WPP lost BSkyB late last year when the satellite broadcaster left United and picked WCRS over WPP's Grey.
CHI has billings of around £200m, including founding client Carphone Warehouse and domestic clients such as Argos, British Gas and COI. Its multinational clients include Lexus and Toyota.
Sorrell said: "We have admired Johnny Hornby, Simon Clemmow, Charles Inge, Simon Hall, Warren Moore and their colleagues for many years. We think that the proposed structure gives CHI the chance to capitalise on WPP's resources, while maintaining its independence."
Back in February, Vincent Bollore, the Havas chairman, was confident he would land the CHI deal, saying: "CHI is a very attractive and profitable company and we believe we could be a good partner for it. However, nothing has been signed and we are still some weeks away from any agreement."
CHI was also believed to have been talking to Dentsu and other global marketing services groups.