The purchasing consortium consists of The Carlyle Group, Stone Point Capital LLC, Pictet & Cie and Edmond de Rothschild Group.
The group will pay $15.55 a share, a 19% premium to the $13.05 closing level of Duff & Phelps' shares on Friday.
In a news release, Duff & Phelps said the deal was expected to close in the first half of 2013.Noah Gottdiener, chief executive officer of Duff & Phelps, said the transaction was in the best interest of shareholders, who receive "an immediate and certain cash premium for their shares." Gottdiener also said the deal would be structured to "preserve the firm's independence." New York-based Duff & Phelps provides financial advisory and investment banking services. The deal provides for a "go-shop" period -- ending Feb. 8 -- during which Duff & Phelps can seek superior offers from other potential suitors. There is a $6.65 million breakup fee if Duff & Phelps cancels the deal because of a superior proposal made during the "go-shop" period.