The degree of gelatinization of potato starch was measured by the apparent optical density of starch-water suspensions at 530nm during isothermal gelatinization at 52.5-72.0°C. From the temperature dependence of the gelatinization rate, the activation energy of gelatinization was determined to be 22±6 kcal/mol, being in good agreement with the value obtained by the enzymic digestion method in the previous paper. The gelatinization temperature as measured from the halftransition was found to be 61.1°C which is slightly higher than the value of 59.1°C determined by the glucoamylase method. Fromthe transition curve, the van't Hoff enthalpy was evaluated to be 117±3 kcal/mol.