This paper presents a novel data processing method for thermokinetics of first-order reactions, in which the rate constant of a first-order reaction can be calculated from only three characteristic peak-height data without using any peak-area, rate of temperature rise, or functional table of dimensionless parameters. The saponification of ethyl benzoate and ethyl acetate in aqueous ethanol have been studied with a twin conduction calorimeter and the rate constants calculated with this method are in fair agreement with those in the literature; hence the validity of this method is demonstrated.