Volume changes at a mercury cathode in hydrogen-saturated aqueous 0.1 M hydrochloric acid during electrolysis at 25 °C are less than those predicted by Faraday's Law[Formula: see text]over a range of current densities from 0.4 to 83 μA mm−2. Difficulties attributable to the anodic reaction of mercury were found using an apparatus similar to that of Müller (1) but constructed entirely of glass. A two-compartment cell proved to be advantageous in isolating the cathode reaction. Rates of volume change attributable to formation of H2(g) almost invariably increased with time of electrolysis but were always below expectation. In agreement with Müller, an increase of volume was observed after electrolysis. The evidence is consistent with formation and subsequent decomposition of a hydrogen amalgam. It suggests that, contrary to the usual assumptions (2), hydronium-ion discharge is not the slowest step in the overall hydrogen gas-producing reaction.