The thermal evolution of the reference catalyst, EUROCAT EL10V8, containing 8% (w/w) of V2O5 deposited on anatase TiO2 has been followed by XRD, chemical analysis and XPS during annealing in oxygen for periods from 0 to 288 h and at temperatures from 420 degrees C to 640 degrees C, The anatase structure of the support is stable up to 520 degrees C. Above this temperature the rutile structure appears. The amount of NH3-insoluble vanadium, formed at 300 degrees C, increases with the time and temperature of annealing from 420 degrees C onwards. Below 560 degrees C the fraction of the TiO2 surface covered by vanadium oxide, as determined by XPS, falls with increasing time, This evolution is accelerated by water vapour. From 560 degrees C the specific surface decreases with the time, Half the insoluble vanadium is bound between the surface and a depth which increases from 2.5 Angstrom in the initial state of the catalyst up to about 28 Angstrom after 48 h annealing at 560 degrees C. At the same time, the extent of vanadium binding in the surface plane of TiO2 decreases from 0.30 atom per unit cell to 0.12.