The transformation of VOHPO(4)(.)0.5H(2)O to (VO)(2)P2O7 has been studied in the absence and presence of water vapor (0-40 vol%) using small (340 urn x 40 nm) and large (10,000 nm x 410 nm) VOHPO(4)(.)0.5H(2)O crystallites. The small VOHPO(4)(.)0.5H(2)O crystallites were transformed into a single-phase of well-crystallized (VO)(2)P2O7 within 5 h under a reactant gas (0.9% n-butane, 10% O-2 and He(balance)) containing 40% water vapor, whereas the transformation took more than 100 h in reactant gas without water vapor. In the large-sized VOHPO(4)(.)0.5H(2)O crystallites, (VO)(2)P2O7 was the main phase formed in the presence of reactant gas containing 40% water vapor, whereas alpha(II)-VOPO4 was the main product in the absence of water vapor. It was found that, under the reaction conditions, water vapor accelerated two processes in the transformation of VOHPO(4)(.)0.5H(2)O to (VO)(2)P2O7; the crystallization of the amorphous VP phase containing V4+ and V5+ to (VO)(2)P2O7 and delta-VOPO4, and the transformation of delta-VOPO4 to (VO)(2)P(2)O7, On the other hand, water vapor inhibited the topotactic transformation of VOHPO(4)(.)0.5H(2)O to (VO)(2)P2O7. The two processes accelerated by water vapor resulted in the rapid transformation of VOHPO(4)(.)0.5H(2)O to (VO)(2)P2O7 under the reactant gas. The catalyst obtained in the presence of water vapor gave stationary conversion and selectivity for MA from immediately after the beginning of the reaction. This catalyst showed a high selectivity for MA in comparison to the catalyst transformed in the absence of water vapor. Furthermore, over the catalyst formed in the absence of water vapor, it took more than 100 h to reach stationary conversion and selectivity. (c) 2005 Elsevier B.V. All rights reserved.