A major microcalorimetric problem, the development of a very sensitive thermometer, is considered. The theory of a pyroelectric thermometer is reviewed and verified calorimetrically. A thermostatic system utilizing both a layer of low thermal diffusivity material and a large heat sink was used to strongly attenuate external temperature fluctuations. A pyroelectric thermometer, used in the thermostat, measured temperature changes as large as 2.52×10−3°C and as small as 6×10−6°C. The sensitivity of the thermometer was limited by the temperature stability of the thermostat. Technically simple modifications of the calorimeter could make the detection of temperature changes as small as 5×10−7°C possible. Detection of temperature changes smaller than 10−7°C may be achieved by changes in size, shape, and material of the pyroelectric sensing element.