Silica-supported Pd catalysts were prepared through non-aqueous impregnation of silica with palladium acetylacetonate [Pd(acac)(2)]. The influence of the pretreatment atmosphere (O-2, H-2, CO, N-2 and vacuum) at 570K on the decomposition of the metal complex, and on the oxidation state of the resulting supported Pd particles, was investigated by FTIR spectroscopy of adsorbed CO (at 298 K). Under a mildly reducing (CO) atmosphere Pd-0 metal particles were formed, whereas under oxidising (O-2) conditions Pd2+ was also detected. Reduction under H-2 caused the formation of a carbide-like deposit on the silica surface which blocked all Pd sites to CO adsorption. Pretreatment in N-2 and under static vacuum led to decomposition of the acac Ligands and subsequent reduction of Pd by evolved reducing components. This resulted in formation of surface Pd species of mixed valence in the former, and dispersed Pdo particles in the latter sample, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.