The dependence on temperature of the voltammetric behavior of Pt(111), Pt(100), and Pt(110) electrodes in 0.1 M HClO4 has been studied. A thermodynamic analysis of the hydrogen underpotential deposition (H-UPD) has been carried out, taking into account that the adsorption process is accompanied by changes in the electrode double layer. The analysis for Pt(100) requires a previous deconvolution of the OH adsorption contribution whereas for Pt(110), the analysis is limited to high H coverages. The Pt-H bond energy values obtained for the Pt(hkl) electrodes agree with the values obtained in UHV. On Pt(111) and Pt(100), DeltaGdegrees(ads)(H-UPD) changes ad linearly with coverage, with Frumkin repulsive parameters 27 and 9 kJ mol(-1), respectively. The values obtained for DeltaSdegrees(ads)(H-UPD) (-48 J mol(-1) K-1 for Pt(111), - 56 J mol(-1) K-1 for Pt(100), and from -55 to - 70 J mol(-1) K-1 for Pt(110)) suggest immobile hydrogen adsorption. The fact that DeltaSdegrees(ads) (H-UPD) depends significantly on ad the crystallographic orientation suggests that the symmetry of the platinum substrate strongly influences the degree of order in the water network directly bonded to the metal surface atoms.