The biological upgrade of biogas to biomethane (CH4 > 90%) is a popular emerging technology, since the produced CO2 is not removed but is converted to biomethane using hydrogen (H2). The initial aim of the present study is the acclimatization of a microbial population derived from a typical biogas plant, under high concentrations of H2, as well as the development of an enriched biomass in hydrogenotrophic methanogens. The enriched biomass was used to inoculate ex-situ bioreactors for biomethane production under continuous operation. Specifically, a bubble reactor and a trickle bed reactor were studied, and their performance was compared under the same operating conditions. Both reactors were provided with a mixture of H2, CH4 and CO2, which was injected through a conventional diffuser. The methane content in the upgraded biogas reached 92.7±1.1% and 95.7±1.1% for bubble and trickle bed reactor respectively, under a loading rate of 1.26 LH2 Lreactor-1 d-1 (11.5h gas retention time). However, at increased H2 loading rates the trickling bed reactor outperformed the bubble reactor.