A reassessment of the basic equation governing calorimetry shows the crucial importance of temperature stability in the calorimeter environment. Following the principles derived, we have developed a new temperature control method and demonstrated it in a prototype calorimeter over a wide dynamic range (5 &mgr;W–2 W). Temperature control is achieved by using a metal block of high thermal inertia, surrounded by heat flow sensing thermopile arrays, which is embedded in an isothermal environment. Any temperature fluctuations in this environment are detected by the heat flow sensors and can be corrected by conventional feedback control. The advantage of this method is its high sensitivity to temperature changes: whereas conventional thermometers typically provide signals in the order of 1×10−3V/K−1, a correctly selected combination of inertial mass and thermopile arrays can provide signals in the order of 1 V K−1, thus permitting an improvement of temperature control by three orders of magnitude. A commercially available thermoelectric calorimeter in an isothermal environment controlled by the new method permitted measurement of sample powers with ±10 &mgr;W accuracy (long‐term, short‐term noise ∼±1 &mgr;W), whereas in a conventional thermostatic bath (temperature stability ±0.02 K h−1) its resolution was limited to ±3 mW.