Cyclic voltammetry was performed with a nickel electrode in molten CsH<sub>2</sub>PO<sub>4</sub> at 350 °C and at approximately 25 bar of water pressure. The effect of adding carbon dioxide was examined. It is found that even small amounts of CO<sub>2</sub> (partial pressure below 0.2 bar at 350 °C, corresponding to 0.5 bar at 22 °C of a 5:1 mixture of N2:CO<sub>2</sub>) have a pronounced effect on the cyclic voltammetry curves. Under these conditions the hydrogen formation is substituted by another process, methane formation. At the same time the cathodic current increased by a factor of 4–6, meaning that the ease of methanisation was much improved.