The effects of several metal ions and metal ion complexes on the hydrolysis and interconversion of uridylyl(2',5')uridine and its 3',5'-isomer were studied as a function of pH and metal ion concentration. The hydrolysis was shown to be markedly accelerated by Zn2+, Cd2+ and trivalent lanthanide ions and by tri- and tetraaza complexes of Zn2+. In contrast, none of these species appreciably promotes the interconversion of the 2',5'- and 3',5'-isomers, in spite of the fact that this reaction proceeds through the same pentacoordinated intermediate as the hydrolysis. Lanthanide ions also promote the hydrolytic dephosphorylation of uridine 2'- and 3'-monophosphates, but have a barely noticeable effect on their interconversion. The mechanisms of the metal ion-promoted reactions are discussed.