The thermal evolution of the reference catalyst, EUROCAT EL10V8, containing 8% (w/w) of V 2 O 5 deposited on anatase TiO 2 has been followed by XRD, chemical analysis and XPS during annealing in oxygen for periods from 0 to 288h and at temperatures from 420 o C to 640 o C. The anatase structure of the support is stable up to 520 o C. Above this temperature the rutile structure appears. The amount of NH 3 -insoluble vanadium, formed at 300 o C, increases with the time and temperature of annealing from 420 o C onwards. Below 560 o C the fraction of the TiO 2 surface covered by vanadium oxide, as determined by XPS, falls with increasing time. This evolution is accelerated by water vapour. From 560 o 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 9 in the initial state of the catalyst up to about 28 9 after 48h annealing at 560 o C. At the same time, the extent of vanadium binding in the surface plane of TiO 2 decreases from 0.30 atom per unit cell to 0.12.