The effect of CH2O, CH3OH, and C2H5OH on the methane conversion and the CO, CO2, and coke yield in the methane steam conversion on commercial nickel catalysts for steam reforming C 11-9-09 (12.8 wt % Ni/alpha(x-Al2O3) and hydrogenation (54.0 wt % Ni/kieselguhr) was studied at a temperature of 750 degrees C and an H2O : CH4 ratio of 1.5-2.0. The action of these compounds was studied both individually and for their joint presence in the methane-steam mixture. Their effect on the reaction depends on the pore structure of the catalyst. Using the catalyst C 11-9-09 as an example, it was found that the introduction of formaldehyde into steam suppressed the methane conversion into C-s and high-boiling-point carbon compounds and gave the desired products with a yield close to the equilibrium value. When all three additives were present in the steam, ethanol was responsible for the formation of C-s.