The influence of crystallite size of precursor VOHPO(4)(.)0.5H(2)O on the microstructure of a resulting catalyst and selective oxidation of n-butane are investigated. Two kinds of VOHPO(4)(.)0.5H(2)O, small crystallites (av. 1 mum x 110 nm) and large crystallites (av. 10 mum x 415 nm), were prepared by intercalation-exfoliation-reduction of VOPO(4)(.)2H(2)O in 2-butanol and the direct reduction of VOPO(4)(.)2H(2)O with 2-butanol, respectively. The small VOHPO(4)(.)0.5H(2)O crystallites transformed into single-phase (VO)(2)P2O7 of high specific surface area under the reaction conditions of 1.5% n-butane, 17% O-2, and He (balance) at 663 K. In contrast, the catalyst formed from the large VOHPO(4)(.)0.5H(2)O crystallites assumed the form of particles having a double-layered structure, consisting of peripheral (VO)(2)P2O7 and internal alpha(11)-VOPO4. As compared with the large (VO)(2)P2O7, the small (VO)(2)P2O7 Showed high selectivity to maleic anhydride (similar to78% at 663 K) and higher catalytic activity. Meanwhile, the catalyst with the double-layered structure exhibited moderate selectivity (similar to70%). Since (VO)(2)P2O7 is considered to be a selective phase for the selective oxidation of n-butane, the single-phase of (VO)(2)P2O7 derived from the small-sized VOHPO(4)(.)0.5H(2)O crystallites is considered a reason for the high selectivity to maleic anhydride. (C) 2004 Elsevier B.V. All rights reserved.