An absolute method for calibrating isothermal microcalorimeters is described which uses a well-characterized, physicothermal property of a stable, pure material of known mass. The technique exploits the loss or gain of heat from a sample of the specific-heat standard, alumina, following a small, but rapid change in the operating temperature of the calorimeter The results obtained using this specific-heat calibration method are compared with calibrations made using electrically heated calibration devices.