5YO(1.5)-10CeO(2)-85ZrO(2) solid solution doped with vanadium was studied by means of electron paramagnetic resonance (EPR). The nature of the vanadium species formed by treatment of samples with O-2, CO, and H-2 was investigated. Three types of paramagnetic V4+ ions found in samples evacuated at 723 K have been attributed to tetragonally compressed octahedral complexes of vanadyl type with different interionic V-O distances. Two of them are supposed to be located on the surface of the oxide support, whereas the third type is in the bulk of the solid. EPR spectroscopy of all CO reduced samples showed the presence of isolated mononuclear V4+ species. Reducing treatment of the samples with H-2 also provoked the formation of aggregated V4+ species. The later species formed either by the agglomeration of surface isolated V4+ ions during high-temperature treatment or the reduction of polymerized V5+ species present in as-prepared samples. Obtained results can be used for the better understanding of vanadium effect on catalysts properties. (C) 2003 Kluwer Academic Publishers.