A Borelius type isothermal jacket microcalorimeter has been designed and constructed. It consists essentially of a vapor thermostat providing a constant temperature environment about a chamber containing the specimen and measuring device, the latter being a high‐sensitivity differential thermopile. With one set of thermopile junctions kept at constant temperature by contact with the environment, and the other set arranged to vary in temperature with the specimen, a heat effect in the specimen is measured by following the difference in temperature across the thermopile as a function of time. Calibration of the apparatus is carried out by using the Peltier heat developed at the junction of a thermocouple located in an axial hole in the specimen. With an argon atmosphere in the specimen chamber, the limit of detection is a heat flow of 0.003 gram‐calories per hour. The absolute accuracy of the determinations depends upon the particular conditions of a given run, but is usually in the vicinity of 2 to 5 percent.