Platinum nanoparticles supported on porous silicon were synthesized by radiation chemi cal reduction in solutions of reverse micelles. The Pt nanoparticles obtained are electron deficient. The degree of porosity, conductivity type, pore geometry of the silicon matrix, and precursor parameters affect the size, shape, and charge state of the platinum catalysts.Platinum and platinum based composites are consid ered to be the best catalysts of oxygen reduction reactions (ORR) and hydrogen oxidation reactions (HOR) that occur in electrochemical power generators. It was shown in a series of our works 1-4 and other publications 5-8 that nanoparticles of platinum metals and silver supported on porous silicon (PS) and carbon nanotubes are efficient electrocatalysts for fuel cells.Synthesis of metal nanoparticles in solutions of reverse micelles followed by their subsequent adsorption on po rous supports is a promising method for nanoelectrocata lyst formation. 2-4 Reverse micelles are spherical micro droplets of water (pools) stabilized by surfactants (Surf) in an organic solvent. The molar ratio ω 0 = [H 2 O]/[Surf] defined as the solubilization coefficient determines the size of formed metal nanoparticles during the reduction of their ions in water pools. An increase in this ratio favors the formation of larger nanoparticles.There are few published data describing how the elec trocatalytic activity of nanoparticles deposited on the PS surface is influenced by their physicochemical proper ties. 2-6 Therefore, it is of interest to reveal the character of the metal-PS interaction, morphology of supported particles, charge state of the surface, and possibility of formation of new phases and active sites. Information of this kind makes it possible to better understand specific features of the mechanism of electrocatalytic reactions.The formation of catalytic composites involving Pt nanoparticles can result in the appearance of electron deficient Pt δ+ atoms and the electron density redistribu tion at the Pt/semiconductor interface. In addition, chemi sorption of metal nanoparticles on the support will lead, probably, to the formation of chemical compounds be tween the catalyst and supporting matrix. 9,10 On the one hand, it is considered 11 that the adhesion of Pt nanoparti cles on the silicon support is weak because of the low surface energy of interaction between Pt and Si. On the other hand, it is elucidated that new crystalline phases based on Pt, Re, Ru, and Mo are formed 12,13 during the formation of supported metal catalysts with high degree of dispersity. The high electrocatalytic activity of the nano catalyst Pt 3 Sn was shown 14 in the reaction of EtOH oxi dation. Surface intermetallic compounds Pt 3 Co, Pt 3 CoCr, and Pt x Ni are active in the ORR. 15, 16 To solve many controversial questions about the inter action of metal particles with the support, it is important to know the state of these particles on the surface.Measurements of surface potential are usually used to determine the sign of charge of adsorbed ca...