The site occupation of atomic hydrogen in the topmost atomic layers of fuel cell catalysts was studied. Platinum and platinum/ruthenium particles of varying dispersity supported on carbon black and unsupported particles of platinum black were investigated by means of inelastic neutron scattering. The materials were hydrogenated inside of in situ neutron cells and were measured under frozen hydrogen sorption equilibrium. Transmission electron microscopy revealed the stability of the dispersion of the supported precious metals to the alternating in situ hydrogenation/dehydrogenation cycles. With increasing particle size the vibrational mode of atomic hydrogen on nanodisperse platinum in the range of 500-600 cm(-1) was narrowed and its maximum signal shifted down to lower energies. The H occupation of (111) terraces of the platinum particles increased. The relative intensities of the INS scattering contributions from C-4v sites at about 460 and 650 cm(-1) decreased. The most prominent spectral changes were observed in the transition region around 3.0-4.8 nm of the average primary particle size of the precious metal which according to Kinoshita (J. Electrochem. Soc. 137 (1990) 845) is correlated to specific electrocatalytic activity. It was possible to discriminate between the vibrational modes of hydrogen atoms on different Surface sites on platinum, Pt-OH groups, Pt-x/Ru-y-OH groups, and traces of water. The Pt-OH translational modes ranged from 200 to 400 cm(-1). The Pt-OH bending modes were observed at 840, 950, and 1016 cm(-1) for platinum black and at 810, 877, and 954 cm(-1) for Supported Pt/Ru particles exhibiting also Pt2Ru and PtRu2 sites. Water and Pt-OH groups are formed during the first interactions between gaseous hydrogen and the catalyst surface. The extraction of residual traces of oxygen from larger precious metal particles by hydrogen cycling procedures seemed to be limited by diffusion phenomena. This was of minor relevance for nanodisperse platinum species. (C) 2004 Elsevier Inc. All rights reserved.