An improved method for synthesizing a double metal cyanide (DMC) catalyst is disclosed. The method combines an aqueous solution of a first metal salt, such as ZnCl.sub.2, with an aqueous solution of a second metal salt, such as CoCl.sub.2, and with an aqueous solution of an alkali metal cyanide, such as KCN, in a single step to synthesize the DMC catalyst, Zn.sub.3 [Co(CN).sub.6 ].sub.2.