The cleavage and isomerization of 3’,5’-uridyluridine catalyzed by morpholine buffers and by imidazole buffers has been reinvestigated, The key evidence for a previously proposed partitioning mechanism-in which the buffer acid BH+ converts the substrate to a phosphorane monoanion intermediate which then partitions either to the 2’,5’ isomer of the substrate or (with buffer base catalysis) to the cleavage product-is confirmed. The negative catalytic effect of the buffer base on the isomerization reaction is not due to a medium effect. Indeed the medium effect on this reaction is in the opposite direction, strengthening the catalytic evidence. However, this branching mechanism with sequential bifunctional catalysis of the cleavage reaction is accompanied by an additional cleavage path using. the buffer base only, This additional path, for which several alternative mechanisms are possible, is required by the results of improved studies on the imidazole catalysis. These show that the previously reported decrease in rate at a low BH+/B ratio is due to ionic strength effects, The relative importance of these two pathways-one with kinetic dependence on the buffer acid and one without such dependence-depends on the buffer basicity/acidity. With the acidic buffer acetate/acetic acid, a buffer-acid-catalyzed mechanism for the cleavage and the isomerization is dominant, A bifunctional mechanism, in which one step involves simultaneous acid-base catalysis, seems most likely, The medium effects of added dioxane on all these reactions are sensible in terms of the detailed mechanisms proposed, The relationship of these results to the mechanisms of catalysis by ribonuclease and by an enzyme mimic is discussed.