Experiments have been carried out in an industrial scale pilot plant reactor and a microreactor to investigate the reversible/irreversible deactivation of a V20~/Ti02 catalyst used for the partial oxidation of o-xylene to phthalic anhydride. The experimental results indicate that unless the catalyst experiences a very high temperature for a long period, the deactivation is reversible and may be caused by the deposition of carbonaceous compounds on the surface. A simplified expression for the kinetics of deactivation and reactivation has been developed. This has been used, together with a reactor model, to estimate on-line activity profiles using both early and late lumped filters. The theoretical derivation shows that the early lumping preserves the full estimate error weighting matrix, and is identical to the late lumping. Using three real-time temperature measurements, the catalyst activity profile is accurately inferred and the dynamic behaviour of the reactor is excellently tracked. Computational experience indicates that the estimator can be implemented on-line for the control and optimisation of the reactor. The results could give valuable guidance on when a catalyst in a reactor should be regenerated or replaced.