Effects of catalyst poisons on the kinetics of the cathodic hydrogen evolution reaction are of interest in relation to changes in coverage by the H intermediate and coupled changes of reaction mechanism. Also, the effects of poisons are of practical significance in promoting sorption of H into transition metals and inhibiting corrosion. Experiments were carried out on the effects of As species on the kinetics of H-2 evolution at Pt electrodes in aqueous KOH as a model system where quantitative information can be derived on coverage by the overpotential-deposited (OPD) H that is the intermediate in the H-2 evolution reaction. Arsenic poison effects on the states of underpotential-deposited H by means of cyclic voltammetry were also discussed. For the OPD H, recording and analysis of open-circuit potential relaxation transients in the absence and presence of As poison species provides information on the H coverage and the effects of the co-adsorbed poison on it. Kinetic simulation calculations provide quantitative estimates of the changes in rate constants of reaction steps arising from the presence of the poison.