The competitive adsorption of catalyst poisons and adsorbed H is of interest in relation to their effects in promoting H absorption into transition metals. The capacity of three sulfur-containing compounds, thiourea, L-cysteine and 2,2′-diethanolsulfide (DES), to block H adsorption at Pt electrodes in both the underpotential deposition (UPD) and overpotential deposition (OPD) regions is studied. Differences in the rates of adsorption (cysteine and DES < thiourea) and the reactivity of these molecules (thiourea > cysteine and DES) are shown to play important roles in the extent to which the poison, P, can block the UPD of H and modify the Tafel relationship for the hydrogen evolution reaction (HER). The relative H blocking, due to poison coverage, θ_p, is measured by means of cyclic voltammetry and potential-relaxation transient experiments. Measurements of rates of the HER as a function of overpotential are also made utilizing steady-state techniques. It is shown that, for θ_p of cysteine and DES < 0.9, inclusion of a constant site-blockage term is sufficient to account for the observed kinetic behaviour. In the presence of adsorbed thiourea, and virtually complete initial coverages of cysteine and DES, i.e. before any H₂ evolution takes place, θ_p varies with potential according to an isotherm determined by the kinetics of hydrogenation and/or desorption of the poison.