The hydrolysis of uridylyl(3',5')uridine promoted by Mg2+, Zn2+ and Zn2+([12]aneN(3) = 1,5,9-triazacyclododecane) has been studied in imidazole, HEPES and triethanolamine dagger buffers and the origin of marked electrolyte effects observed with Zn2+[12]aneN(3) has been examined. The results obtained suggest that the basic buffer constituent does not serve as an external general base, but the catalytic activity of the metal ion species is influenced by the coordination to the buffer base and other Lewis bases present in the solution. The data lend additional support to a bifunctional mechanism, which consist of coordination of the metal ion to the anionic phosphodiester and intracomplex general base (or nucleophilic) catalysis by its hydroxo ligand.