This paper deals with a new quasi-isothermal method for the measurement of heat capacities of pure solids and liquids in an LKB 2277 heat flow microcalorimeter TAM (thermal activity monitor). Deviations of less than 1% from literature data can be achieved. It is shown that even excess heat capacities of binary mixtures can be determined. With a quasi-isothermal method, no heating up or cooling down of the calorimetric unit is needed; thus the temperature of the thermostat directly represents the test temperature.