Hydrogen evolution reaction kinetics were investigated on a rotating silver disc electrode in solutions of citric acid/citrate anions and different total concentrations of the organic and different pH. The aim was to establish whether hydrogen atoms bound in the undissociated acid and in the acid anions participate in the reaction as hydrogen donors. The galvanostatic pulse method was used to establish Tafel plots over a current density range between 0.1 and 100 mA cm(-2). Diffusion limiting currents proportional to the total concentration of the organic are found pointing to the participation of the latter in the process. In order to distinguish between (a) evolution of hydrogen via hydronium ions arising from dissociation of the citric acid or acid anions, and (b) direct involvement of hydrogen bound in those species, a diagnostic criterion is found in the pH dependence of the current density at a constant potential. The analysis resulted in the conclusion that mechanism (b) is operative.