Hydrogen evolution on Pt(111) is investigated as a function of temperature; the results are interpreted in terms of the theory of electrocatalysis. The current-potential curves are consistent with a Volmer-Tafel mechanism, in which the recombination of two weakly adsorbed hydrogen atoms determines the overall rate, and in which the adsorbed hydrogen shows a repulsive interaction. The pre-exponential factor of about 10(9)A cm(-2) is high and indicates a rate determining step that occurs only on the surface. The activation energy is of the order of 0.5 - 0.6 eV and concurs with previous theoretical estimates. (C) 2017 Elsevier Ltd. All rights reserved.