The evolution of surface species on unsupported V-O-W catalyst (V:W = 2:9) as a result of its oxidizing and reducing thermal treatments was investigated. The catalyst was prepared by annealing an oxalate precursor at 773 K in air for I It and then subjected to further oxidizing or reducing thermal treatments. It was revealed using XRD that the freshly prepared catalyst contains mainly the crystallites of the tetragonal phase of V-W oxide bronze which transforms into monoclinic WO3 during further annealing due to surface vanadium segregation. XPS results showed that in the surface nanolayers of the freshly prepared catalyst vanadium occurs mostly as vanadium suboxides species. Raman spectroscopy results revealed mostly crystalline vanadia-like species as well as monolayer vanadia-like species on the freshly prepared catalyst. Essential changes in the structure of the vanadia-like species as a result of the catalyst thermal treatments were observed. The thermal treatment in the oxidizing atmosphere caused an increase in the content of the crystalline vanadia-derived surface species-that did not contain tungsten in their structure. Monolayer species with relatively high tungsten content were formed during the catalyst thermal treatment in reducing conditions. (c) 2006 Elsevier B.V. All rights reserved.