A method is suggested for measuring the velocity constants of rapid reactions in solution by observing the maximum or minimum temperature reached when the reaction takes place under non-adiabatic conditions. Theoretical expressions are derived relating the temperature change to the thermal constants of the system and to the velocity constants of reactions of zero, first and second order. A simple apparatus is described for applying this method, the temperature changes involved being less than 0$\cdot $1 degrees C, so that the variation of reaction velocity with temperature can be neglected. In order to test the method experimental data are given at 0 and 25 degrees C for the following reactions of the first and second order: the alkaline hydrolysis of methyl acetate and of ethyl glycollate, the acid-catalyzed hydrolysis of diethyl acetal, and the reaction of nitroethane with hydroxyl ions. There are no systematic deviations from the theoretical relations. The internal consistency of the results and their agreement with other data show that velocity constants can be measured with an accuracy of 2 to 3% for reactions with half-times in the range 3 s to 3 min.