Temperature programmed desorption experiment of adsorbed hydrogen was carried out over UHV-treated Pt powder catalysts, that is Pt powder evacuated at high temperatures under ultrahigh vacuum (less than or equal to 10(-7) Pa), Platinum powder evacuated at 673 K under conventional vacuum (approximate to 10(-2) Pa) indicated two desorption peaks at 130 and 190 K. The peaks observed at 130 and 190-K-were assigned to associatively and dissociatively adsorbed hydrogen species, respectively, by TPD experiment of co-adsorbed H-2-D-2. The Pt powder UHV-treated at 1073 K showed a new desorption peak which was assigned to dissociatively adsorbed hydrogen species at 160 K in addition to the peaks at 130 and 190 K observed on the Pt powder evacuated under 10(-2) Pa. The fourth peak was observed at 250 K on Pt powder prepared by direct decomposition of hexachloroplatinic acid, which was a starting material of the Pt powder, at 673 and 1073 K under UHV. These new peaks at 160 and 250 K were also observed in the case of adsorption even at 77 K. Molecular hydrogen can be adsorbed dissociatively and activated on the Pt powder prepared by decomposition of hexachloroplatinic acid. The UHV-treated Pt powder catalysts at 1073 K indicated high activity for H-2-D-2 exchange reaction, Furthermore, the Pt powder prepared by the decomposition at 1073 K showed markedly high activity even at a reaction temperature of 77 K. These results clearly indicate that the UHV treatment generates new active sites for activation of hydrogen molecules.