Cobalt-impregnated VPO catalysts containing 1-6% of the metal by weight were prepared. The catalytic tests showed that cobalt impregnation significantly increased the overall activity while slightly decreasing the maleic anhydride selectivity To investigate the origin of the cobalt effect, the solids were characterized using XRD, Raman spectroscopy, FT-IR, TPR and XPS. No structural effects were detected through XRD. TPR showed that the non-equilibrated catalysts contain cobalt oxides which react with the VPO solids to yield a hard to reduce system. Non-equilibrated umpromoted and promoted (low cobalt loadings) catalysts contain small proportions of V(V) phases. After several 100 h on stream, the only phase detected in all cases was V(IV) vanadyl pyrophosphate. The surface oxidation state of vanadium was V(IV). The maleic anhydride yield correlates with the increased exposure of cobalt on the catalysts surface. (C)2000 Elsevier Science B.V. All rights reserved.