The influence of the tribomechanical pretreatment of the precursor vanadyl hydrogenphosphate hemihydrate, VOHPO4. 0.5H(2)O, prepared in aqueous medium and of the catalyst vanadyl pyrophosphate, (VO)(2)P2O7, on the structural and catalytic properties of (VO)(2)P2O7 in the oxidation of n-butane to maleic anhydride was investigated. Due to the pretreatment the activity and selectivity of the catalyst were markedly increased. The increase in catalytic performance is explained as an effect of the particle size in connection with lattice imperfections.