In situ biogas upgrading was conducted by introducing H(2) directly to the anaerobic reactor. As H(2) addition is associated with consumption of the CO(2) in the biogas reactor, pH increased to higher than 8.0 when manure alone was used as substrate. By co-digestion of manure with acidic whey, the pH in the anaerobic reactor with the addition of hydrogen could be maintained below 8.0, which did not have inhibition to the anaerobic process. The H(2) distribution systems (diffusers with different pore sizes) and liquid mixing intensities were demonstrated to affect the gas-liquid mass transfer of H(2) and the biogas composition. The best biogas composition (75:6.6:18.4) was obtained at stirring speed 150 rpm and using ceramic diffuser, while the biogas in the control reactor consisted of CH(4) and CO(2) at a ratio of 55:45. The consumed hydrogen was almost completely converted to CH(4), and there was no significant accumulation of VFA in the effluent. The study showed that addition of hydrogen had positive effect on the methanogenesis, but had no obvious effect on the acetogenesis. Both hydrogenotrophic methanogenic activity and the concentration of coenzyme F(420) involved in methanogenesis were increased. The archaeal community was also altered with the addition of hydrogen, and a Methanothermobacter thermautotrophicus related band appeared in a denaturing gradient gel electrophoresis gel from the sample of the reactor with hydrogen addition. Though the addition of hydrogen increased the dissolved hydrogen concentration, the degradation of propionate was still thermodynamically feasible at the reactor conditions.