The effect of addition of cobalt to an amorphous vanadium phosphate for the selective oxidation of n-butane to maleic anhydride is described and discussed. Cobalt is a well known promoter for crystalline vanadium phosphate catalysts and is most effective at a concentration of 1 atom % relative to vanadium. In contrast, for amorphous vanadium phosphate materials, prepared by precipitation using supercritical CO₂ as an antisolvent, cobalt appears to act as a catalyst poison, decreasing both the catalyst activity and selectivity for maleic anhydride. Detailed analysis by transmission electron microscopy, ³¹P spin echo mapping NMR spectroscopy and X-ray absorption spectroscopy is described, which highlight differences with the unmodified catalyst. It is concluded that the addition of cobalt affects the morphology of the material and the oxidation state of vanadium, and that these changes deleteriously affect the catalytic performance.