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, ΔS0I, to be evaluated. Three of the states have a positive ΔS0I 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.