© 2019 American Chemical Society. The effects of the alcohol concentration of a hot compressed water-alcohol (methanol or ethanol) mixture on acid-catalyzed ester reactions were investigated. In the water and water-methanol mixture, ethyl acetate reactions were promoted by the autocatalytic effect of the produced acetic acid, while diethyl malonate reactions were promoted by the catalytic effect of the solvent molecules. The kinetic analyses indicated that the catalytic effect of H+ derived from the dissociation of acetic acid decreased as methanol concentration increased. The catalytic effect of water molecules was highest in a mixed solvent of 10 mol % methanol, and that of methanol molecules increased as methanol concentration increased. In the water-ethanol mixture, the hydroxyl-decarboxylation of diethyl malonate to ethyl acetate and hydrolysis of ethyl acetate to acetic acid proceeded consecutively. The secondary ethyl acetate hydrolysis was suppressed in the water-ethanol mixture because of the differences in the effects of alcohol concentration on each reaction.