Oxidation of arsenic(III) by the bacterial community of a contaminated sediment (from the Estaque marina, Marseille, France) was studied using microcalorimetry. A low but reproducible heat output was detectable during microbial As(III) oxidation. The thermal effects were of the same order of magnitude as the standard enthalpy for abiotic As(III) oxidation by oxygen. Parameters associated with the biogeochemical cycles of arsenic, iron and carbon were studied in parallel. Amendment with arsenite delayed CO2 production and increased the rate of Fe(II) oxidation in the sediment. These results suggest a correlation between arsenic and iron biogeochemical cycles and mineralization of organic matter.