The temperature dependence of underpotential deposition of H at Pt electrodes has been investigated. The four main states of chemisorbed H which can be distinguished in dilute, highly purified aqueous acid solutions have temperature-dependent standard free energies of electrodeposition and ionization which enable the corresponding standard entropies of adsorption, ΔS⁰_I, to be evaluated. Three of the states have a positive ΔS⁰_I value consistent with discharge from H₃O⁺ to 2-dimensionally mobile states in the Pt surface but with some restriction on mobility due to H₂O and anion adsorption. An intermediate state “H₃” has an almost zero standard entropy of adsorption which can be interpreted only in terms of deposition from water molecules in a special state, e.g. adsorbed and/or bound in the hydration shells of adsorbed anions.

Under alkaline solution conditions, where complications due to anion adsorption are minimized, all states of electrodeposited H have the same entropy which is well accounted for by a 2-dimensional mobile monolayer deposited from water in its normal state.

The significance of apparent decreases in total coverage by H attainable at Pt with increasing temperature is discussed and it is concluded that this effect arises simply because the onset of significant rates of H₂ evolution occurs at more positive potentials (in N₂) as the temperature is raised, thus allowing only a smaller and smaller coverage of adsorbed H to be experimentally measured before H evolution commences, i.e. some of the charge for H deposition must be passed above the H₂ evolution potential for full coverage to be attained at elevated temperatures.