Abstract The initial stage of hydrolysis of gelatin for both Dowex 50 and 1 N HCl catalysis has been studied by viscosity measurements. The activation energies are not significantly different, 19.6 and 20.2 kcal., respectively. Although both processes are very similar with respect to the total concentration of hydrogen ions per unit weight of gelatin substrate, the 1 N HCl hydrolysis proceeds at about 12 times the rate of the Dowex 50 hydrolysis. This difference in rate may be due in part to a decreased hydrogen-ion availability in the case of the incompletely dissociated resin catalyst. Since it has been previously observed that Dowex 50 may catalyze essentially complete hydrolysis of some proteins at rates comparable to HCl, it appears that the resin may be more effective in hydrolyzing the smaller polypeptides occurring during the later stages of hydrolysis. Investigations along this line will be reported later.