The effect of additives on the catalytic performance of vanadium phosphate catalysts is described and discussed in detail for materials prepared using HCl as the reducing agent for V2O2 in water or isobutanol solvents. Performance data are compared on the basis of a simplified kinetic model for n-butane oxidation involving a series-consecutive reaction scheme of pseudo first-order reactions (where C4H2O3 signifies maleic anhydride) : [GRAPHICS] Using this simplified kinetic scheme it is possible to calculate three factors : (a) the rate constant for butane conversion corrected for surface area (k(1) + k(2))/sa which is a measure of the specific activity; (b) the primary selectivity S-0 (selectivity extrapolated to zero conversion); and (c) the relative rates of oxidation of n-butane and maleic anhydride k(3)/(k(1) + k(2)), the latter two factors being a measure of the selectivity of the catalyst. It is proposed that these can be used to determine the relative catalytic performance of promoted catalysts. Hence an improved catalyst would require high specific activity (i.e., a high value of (k(1) + k(2))/sa) together with high maleic anhydride selectivity (i.e., a high value of S-0 and a low value of k(3)/[k(1) + k(2)]) For catalysts prepared by the isobutanol/HCl method the addition of Co is found to promote the specific activity while Mo is found to promote both the specific activity and the selectivity to maleic anhydride at high conversion (>90%). However, for most additives no effect on the specific activity is observed although certain additives are found to act as poisons (Ag, Cs) while others promote the oxidation of maleic anhydride (Cs, Pd, Ru, Zn). The specific activity of catalysts prepared using water/HCl with an additional water-washing step was found to be promoted by the addition of a range of transition metal additives and the effect is described in detail with Co promoted catalysts. The mechanism of the promotion is discussed in terms of structural and electronic effects.