Hydrogen evolution from pyrophosphate solution on silver was studied as a function of pH. Linear Tafel functions with slopes of 0.120 to 0.150 Vdec(-1) were obtained over three orders of magnitude of current density. A diagnostic criterion for participation of hydrogen donors other than the hydronium ions is found in the dependence of polarisation at a constant current density on pH. It was found that all the acid species present, i.e. H4P2O7, H3P2O7-, H2P2O72- and HP2O73-, contribute to the hydrogen evolution current with rate constants/A dm mol(-1) decreasing in order of 4.3 x 10(-4), 5.0 x 10(-7), 4.6 x 10(-7) and 6 x 10(-10) respectively. Each of the species makes a contribution in proportion to its abundance which is strongly pH dependent.