The degree of selectivity obtained in the selective oxidation of o-xylene to phthalic anhydride by V2O5/TiO2 catalysts under industrial conditions is no more than 75-80% : the mechanism of the non-selective oxidation has commanded little interest. Principal features of the reaction and the catalyst are reviewed : the formation of polymeric by-products at low conversion, when the surface is not highly oxidised, is noted, as these will lead to carbon oxides, but at high conversion it is probable that adsorbed intermediate species may undergo side-reactions with reduced V sites, or exposed Ti ions. The resulting strongly-held species can only lead to carbon oxides, so that limiting factors may be (i) degree of completeness of the V2O5 monolayer, and (ii) the rate of re-oxidation of reduced surface.