A microcalorimeter suitable for measuring the energy of easily absorbed nuclear radiations is described. The calorimeter is an isothermal type operating at the temperature of liquid nitrogen and using the rate of vaporization of nitrogen under constant pressure as a measure of the heat input. The calorimeter is fairly simple to construct and to operate and has a short equilibration period. Heat inputs to the calorimeter as low as 7×10−6cal./sec. can be measured with an accuracy of one percent.