The pH-rate profiles were determined for the acidic hydrolysis of some 6-substituted 9-(beta-D-ribofuranosyl) purines. The product analyses indicated that the reactions generally proceed with formation of purine bases as initial products. However, at low oxonium ion concentrations the hydrolysis of the unsubstituted compound yields 4-amino-5-formamidopyrimidine, instead of purine formed in highly acidic solutions. The rate constants for the spontaneous and oxonium ion catalyzed heterolysis of the protonated substrates were calculated from the acidity constants and the observed rate constants. The dependence of the partial rate constants on the polar nature of the 6-substituents are consistent with rate-limiting formation of free purine bases and glycosyl oxocarbenium ions. No anomerization of the substrates was observed during the course of the hydrolysis.