A modified bench scale calorimeter has been employed to determine the heat generated by various microbial strains growing on a range of different substrates, covering degrees of reduction from 3 to 6.13. The results are analyzed, and interpreted in the light of coupled enthalpy and elemental balances. The heat released by the microbial cultures has been found to correlate linearly with other process variables, such as biomass generation and oxygen uptake. The ratio between the heat generated and the biomass formed, the so-called “heat yield” (YQ/x), has been shown both on theoretical and experimental grounds to increase with increasing degree of reduction of the substrate and to decrease with increasing biomass yield. The two effects could be combined into a simple model which permits the amount of heat released per unit of biomass formed to be predicted from the degree of reduction of the substrate as the only independent variable. The ratio between the heat generated and the oxygen taken up was constant at 440 kJ (mol O2)−1 throughout all experiments as expected from theoretical considerations for strongly aerobic processes.