It is well known that during the partial oxidation of o-xylene to phthalic anhydride, on a V2O5/TiO2 (anatase) catalyst under industrial conditions, the catalyst can experience both reversible and irreversible deactivation. Experimental evidence presented here suggests that the reversible deactivation can be attributed to the deposition of some carbonaceous compounds. Experiments were carried out in both a microreactor and an industrial-scale pilot-plant reactor. Catalyst samples from the microreactor were analysed by elemental C-H-N analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). These analyses suggest that the decrease in the disappearance rate of o-xylene at high o-xylene concentrations, which the standard redox model cannot predict, is most likely to be caused by the deposition of carbonaceous compounds rather than by the over-reduction of the catalyst. Two types of reversible deactivation are postulated from the experimental results : (1) by easily removable and (2) by strongly adsorbed carbonaceous compounds.Experiments on the pilot-plant reactor exhibited some unusual dynamic behaviour such as multiple steady-state operation, travelling hot spots and decreased catalyst activity, following an attempted reactivation process at a low temperature. These were all found to be consistent with a model based upon the postulated deactivation mechanism and kinetics; corresponding models based upon constant activity profiles could not reproduce the observed results.