An electron microscopy study of vanadium phosphorus oxide (VPO) catalysts is reported. The catalyst precursor VOHPO4 . 0.5H(2)O was prepared from VOPO4 . 2H(2)O using isobutanol as the reducing agent. The catalyst was then fully activated by heating for ca. 75 h under reaction conditions (385 degrees, GHSV 1000 h(-1), 1.5% butane in air), Scanning electron microscopy studies demonstrated that a topotactic transformation occurs between the precursor and the final catalyst. Transmission electron microscopy experiments revealed that there were three distinctive morphologies present in the final catalyst; namely (i) rosette shaped clusters composed of (VO)(2)P2O7 crystallites, (ii) crystalline platelets of alpha(I)I-VOPO4 and (iii) disordered platelets exhibiting surface patches of(VO)(2)P2O7. This latter morphology corresponds to the disordered remnants of hemihydrate particles which have only partially transformed to (VO)(2)P2O7. The (VO)(2)P2O7 platelets were found to be the majority phase making up more than 95% of the volume fraction.