The reaction energy values for pyrite oxidation by chemolithotrophic leaching bacteria (pure cultures of Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, and mixed cultures of L. ferrooxidans and T. thiooxidans) were calorimetrically measured in batch experiments. The obtained data were compared with the nonconservative values for the corresponding abiotic processes that were calculated from standard enthalpies and chemical analyses of the leaching products. Due to the different abilities of the investigated species to oxidize the sulphur moiety of pyrite, the measured reaction energy values ranged from -1100 to -1600 kJ/mol with an accuracy of 4-16%. In samples with T. ferrooxidans and with the mixed cultures, no significant difference between the calorimetric and theoretical reaction energy values occurred. In contrast, pure cultures of L. ferrooxidans exhibited measured values which were up to 200 kJ/mol lower than the theoretical ones. It is highly unlikely that this difference may be explained by a higher energy conservation efficiency of L. ferrooxidans compared to the one of T. ferrooxidans or T. thiooxidans. Besides use in efficiency studies, the collected data can be used to determine the activity of leaching bacteria in natural biotopes by calorimetric measurements.