Methane production was measured in anoxic slurries of rice field soils that were collected from 16 different sites in China, the Philippines and Italy. The following general pattern was observed. Methane started to increase exponentially right from the beginning of anoxic incubation at positive redox potentials (360–510 mV). The concentrations of H2 and acetate during this first phase allowed exergonic methanogenesis with Gibbs free energies of