To investigate the characteristics of hydrogenotrophic methanogenic (HM, 4H(2) + CO2 -> CH4 + 2H(2)O) mixed cultures, continuous stirred-tank reactors (CSTRs) were operated at various H-2 loading rates (HLR, 2.8-14.0 m(3) H-2/m(3)/d) and hydraulic retention times (HRTs, 5-30 d). The feeding gas was composed of H-2/CO2 (80:20 on volume basis), and HRT was controlled by the continuous pumping of a nutrient medium. At a fixed HLR of 4 kg COD/m(3)/d (1 kg COD = 1.4 M-3 H-2), complete H-2 conversion to CH4 (> 98% on a COD basis) was observed up to the HRT decrease to 6 d. However, at HRT 5 d, the H-2 conversion gradually decreased, reaching 30% within 30 d, and the H-2 content in the head space increased to 70%, which triggered the production of acetate with an increased concentration to 3 g COD/L. When the HLR was increased at a fixed HRT of 10 d, the maximum CH4 production rate of 2.75 m(3) CH4/m(3)/d was achieved at 8 kg COD/m(3)/d. A kinetic analysis conducted using the Monod equation showed that HM cultures had a maximum specific growth rate of 0.18 d(-1) and a half-saturation coefficient of 1.73 g COD/L. Though, successful hydrogenotrophic CH4 conversion was achieved, the broth contained soluble microbial products (SMPs) at a concentration of 80-250 mg/L, which exceeded the effluent regulation standard. Most of the organic matters in SMPs consisted of humic-like and fulvic-like organic matters with a low molecular weight (<1 kDa). (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.