Mercarbide catalyst was studied for the ethoxylation of methanol and 1-heptanol. Mercarbide catalyst is very active for the narrow-range ethoxylation of 1-heptanol. The narrow distribution of ethoxylation units in the ethoxylates is elucidated by the constrained reaction on the mercarbide catalyst. In the proposed surface reaction scheme, the following points are presented.  The relatively basic reactants, ethylene oxide and alcohols or ethoxylates, are adsorbed on the relatively acidic sites of the surface.   The interference results in the decrease of ethoxylation rate in each additional ethylene oxide insertion step, due to a smaller number of the ethylene oxide molecules adsorbed near the growing point of alcohol ethoxylates.  The difference in ethylene oxide insertion rates with the numbers of oxyethylene units in the ethoxylate products causes peaking of the distribution.