Catalysts were prepared containing between 1% and 6% of Co by weight. The cobalt was either added during preparation or impregnated on the dry VPO precursor. The catalytic evaluation showed that the cobalt-impregnated solids gave the highest yields to maleic anhydride. To understand the promoting effect of cobalt, the catalysts were characterized using XRD, FTIR, SEM, TPR, Raman spectroscopy and XPS. The solids obtained through the addition of Co during the synthesis of the VPO precursor exhibit less disordered crystalline phases in the (001) direction. No separate Co-containing phase could be detected through the techniques used. Ln the non-equilibrated catalysts, V(V)-containing phases were detected through Raman spectroscopy. After 500 h on stream under reaction conditions (equilibrated catalysts), vanadyl pyrophosphate was the only crystalline phase detected while V(IV) was the only surface species observed using XPS. XRD, FTIR and SEM coincidentally show that depending on the way cobalt is added the solids obtained develop different structural features.