The apparent rate constants, K_c, for the heterogeneous dilute acid hydrolysis of the difficulty accessible portions of microcrystalline cellulose in 0.3 M, 1.0 M and 1.3M HCl containing 0.00 M, 0.08 M and 0.20 M KCl at 60, 70, 75 and 80°C were determined by the loss-in-weight method, and the activation energy, E_a, calculated from the In K_c versus 1/T curves, where T = absolute temperature. When hydrolysis was carried out in 0.3 M HCl only, a value 25 of keals obtained for E_a. In the presence of KCl as added electrolyte E_a was found to be constant with a average value of 12.6 = 0.5 kcals·mole^-1, and, in the range of concentrations studied, did not depend on the concentration of the added electrolyte. A possible mechanism to account for the low activation energy is presented.