THE formation of pyrite (FeS2), an important factor in determining the global redox balance1, has recently attracted biological interest as a possible direct source of energy for early life2–5. The theory implies that carbon dioxide fixation, in competition with hydrogen formation, can serve as the electron sink for pyrite formation and it seems to be supported by the detection of minute grains of pyrite and iron sulphides inside bacteria5–8. Yet it clashes with the conventional assumption that elemental sulphur or a sulphur equivalent (polysulphide or thiosulphate) is the mandatory oxidant for pyrite formation9,10. It has been stressed that the reaction FeS + H2Sright arrowFeS2 + H2 (with H+ as the oxidant) has "never been observed ... during several years of experimentation"10. Here we report the formation of both pyrite and molecular hydrogen under fastidiously anaerobic conditions in the aqueous system of FeS and H2S.