Based on thermodynamic calculations a discussion was given on the need for operation of the water-gas shift (WGS) reaction with a low H2O/CO ratio when a H-2-separation membrane is combined with the reaction and on the degree of possible formation of C and CH4 as the result of side reactions occurring in the mixture of CO, H2O, CO2 and H-2 under WGS conditions. The behaviour of a number of materials including Fe-Cr, Cu-Zn, Co-Cr and Pt/ZrO2 were examined under the conditions where the ratio of H2O to CO in the feed was low, with and without the presence of H2S in the feed. The experimental results suggested that Pt/ZrO2 has potential for use in these conditions.