Steam reforming of methane mixtures with ethylene (1.4-3.8 vol % C2H4) in the presence of the industrial nickel catalyst S11-9-09 (12.8 wt % Ni/alpha-Al2O3) at 750 degrees C was studied. It was shown that the addition of ethylene to the methane-steam mixture reduces the conversion of methane to carbon oxides and increases coke formation. The unfavorable effect of ethylene on the reaction decreases with an increase in the ratio of the catalytic surface area to the free reaction volume. Presumably, this effect is caused by an increase in the contribution of the heterogeneous component of steam reforming, a change that results in an increase in the rate of the interaction of the formed coke with steam.