A method of preparation of high surface area vanadium phosphorus catalysts for the partial oxidation of n-butane is described that is based on a comminution procedure. The ball milling of catalysts precursors in the presence of dispersants is shown to decrease significantly the mean crystallite size of the VOHPO4.0.5H(2)O from >5 x 10(-6)m to ca. 3.5 x 10(-8) m. A range of dispersants and solvents is described and the use of cyclohexane as solvent and the use of a dispersant based on poly-12-hydroxystearic acid is discussed in detail. These catalyst precursors give final catalysts with high surface area following activation (ca. 40 m(2) g(-1) versus ca. 10 m(2) g(-1) in the absence of the comminution). These catalysts are found to be particularly active for use under fuel rich reaction conditions {[butane] > the higher explosion limit} when high maleic anhydride yields can be obtained. The effect of the addition of promoters is also discussed. In general addition of low levels of La, Ce, Cu and Mo all increase the activity of maleic anhydride formation without any significant effect on selectivity when fuel lean conditions are used {[butane] > lower explosion limit}. However, the effect on activity is much less pronounced when fuel rich conditions are used and in this case the addition of these compounds leads to lower maleic anhydride selectivity.