XPS study V2O5-WO3/TiO2 mixed oxide catalysts for Selective Catalytic Reduction (SCR) of NOx carried out by researchers from ten different laboratories shows good reproducibility of the chemical shift results. The binding energies of the corresponding core level spectra allow us to identify the chemical states of main elements as Ti(IV), V(V) and W(VI). No other oxidation states for these elements were observed both for fresh/used and for crushed/monolith samples. Discrepancy in quantitative data can be proposed to arise from the heterogeneity of their composition as a function of depth. This suggestion is confirmed by SIMS data and ion etching experiments which indicate surface location of V2O5 phase, as well as impurity ones, with respect to TiO2 and WO3 and their redistribution as result of catalyst operation. (C) 2000 Elsevier Science B.V. All rights reserved.