This work is based on calorimetric measurements on batch cultures of ferrous iron-oxidizing thiobacilli and leaching cultures degrading solid substrates in the titrn. vessel under aerobic conditions. It has been demonstrated that heat-flow levels are a direct measure for oxidn. rates of known reactions under non-limiting conditions. The total loss of heat-energy per unit of ferrous iron oxidized is identical under similar incubation conditions with different Thiobacillus ferrooxidans strains and different cell densities in culture suspension. Under limiting conditions a reduced heat loss occurred. This may have been due to either a more effective use of the substrate or to a shift to different redox reactions. Oxygen proved to be a limiting factor in dense bacterial cultures with high substrate concns. in the titrn. vessel. The exptl. duration decreased significantly with the use of increasing cell densities. Therefore cell densities in degrdn. expts. with solid substrates were adjusted to between 5 * 109 and 1 * 1010 cells/mL in short-term expts. T. ferrooxidans is the most important species for short-term leaching expts. Calorimetric measurements allow the best strains for a degrdn. of unknown substrates to be found. The calcd. heat energy is a measure of the amt. of converted substrate. High heat-flow only occurs if catabolic reactions take place and maintenance reactions and phenomena like adhesion of cells to surface do not cause significant loss of heat.