Most of the glucoamylase present in non-pasteurized sake was inactivated during storage at 30°C for about 2 weeks, and a high correlation was observed between residual activities and ethanol concentrations. To clarify this phenomenon, effects of ethanol concentration and storage temperature on the stability of glucoamylase from sake koji were investigated using fresh synthetic sake. In addition, similar studies were also made on acid carboxypeptidase. Following results were obtained. (1) Activities of both enzymes were decreased during storage, and the inactivation rates were higher at higher storage temperature and ethanol concentration. (2) Glucoamylase was inactivated rapidly at temperatures above 30°C, but acid carboxypeptidase was more stable than glucoamylase under these conditions. (3) It was presumed that the inactivation of glucoamylase during storage was caused by synergistic effects of storage temperature and ethanol concentration.