Surface potentials (s.p.) of atomic hydrogen adsorbed at low temperature on clean copper films show wide variations from one film to another. This is believed to reflect varying proportions of different crystal planes. Initial adsorption with a negative s.p. may be followed by a weaker adsorption with a positive effect. The negative s.p. corresponds to hydrogen which can be in equilibrium with gaseous molecular hydrogen. This spontaneous dissociative chemisorption has been studied between 242 and 337 K at pressures up to 60 Torr, using vibrating capacitor s.p. measurements to follow the equilibrium concentration of adatoms. Isosteric heats (typically 40–50 kJ mol^–1) are almost independent of coverage on some films, and a major part of the adsorption can then be described approximately by the Langmuir isotherm for dissociative adsorption.