The nature of the oxidising species in/on a vanadium pentoxide (V2O5) catalyst has been studied using a combination of transient techniques: (i) Temperature programmed desorption (TPD), (ii) Temperature programmed reduction (TPR), (iii) Temperature programmed oxidation (TPO) and (iv) Temperature programmed reaction (TPRn). Chemisorbed oxygen was found not to exist on a fully oxidised V2O5 catalyst by TPD. The TPR in CO over V2O5 catalyst gave three peak maxima at 930, 982 and 1043 K, indicating that three types of kinetically different oxygen states exist in/on the catalyst. Reoxidation of the CO reduced V2O5 catalyst by N2O resulted in the quantitative replacement of the lattice oxygen. A further reduction of the N2O reoxidised catalyst gave a significantly different TPR profile compared to the original material suggesting that a less crystalline material had formed. The presence of phosphorus in (VO)(2)P2O7 was found to labilise the lattice oxygen.