The reaction energy values for pyrite oxidn. 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 expts. The obtained data were compared with the nonconservative values for the corresponding abiotic processes that were calcd. from std. enthalpies and chem. analyses of the leaching products. Due to the different abilities of the investigated species to oxidize the sulfur 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 theor. reaction energy values occurred. In contrast, pure cultures of L. ferrooxidans exhibited measured values which were up to 200 kJ/mol lower than the theor. 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 det. the activity of leaching bacteria in natural biotopes by calorimetric measurements.