The model of anaerobic digestion presented before (Vasiliev et al., 1993. Wat. Resources 20, 633-643; Vavilin et al., 1994. Biores. Techn. 48, 1-8; Vavilin et al., 1997. Wat. Sci. Techn. 36, 531-538) was used to describe biphasic kinetics of methane formation during batch tests of psychrophilic H-2/CO2 consumption by microbiota of tundra soil, and mesophilic glucose degradation by glucose-pregrown inocula. Two possible routes of biphasic methane formation were tested: one with hydrogenotrophic and acetoclastic methanogenesis in the first and second step, respectively. The other route followed acetoclastic methanogenesis in both the first and the second step, taking the potential inhibition of acetoclastic methanogenesis into account. Simulations showed that the hydrogenotrophic and acetoclastic methanogenesis in the first and second steps, respectively, described the system dynamics of mesophilic glucose degradation comparatively well. For psychrophilic H-2/CO2 consumption by tundra soil microflora it was shown that the second option with temporary inhibition of acetoclastic methanogenesis in acidic conditions was more probable. Considered in the model were a direct pH inhibition, with Monod kinetics, for total acetate concentration, or a non-ionized acetate inhibition with Haldane kinetics for non-ionized acetate concentration. Both approaches were able to fit the experimental data rather closely. The kinetic coefficients of hydrogenotrophic methanogens, acetoclastic methanogens and acetogens are presented.