The transformation of nano-sized VOHPO 4 ⋅0.5H 2 O crystallites (340 nm × 40 nm) to (VO) 2 P 2 O 7 was examined. The crystalline structure of VOHPO 4 ⋅0.5H 2 O quickly collapsed to form an oxidized amorphous phase within 1 h (reaction conditions: 1.5% n-butane, 17% O 2 , and 81.5% He at 663 K), followed by gradual crystallization to (VO) 2 P 2 O 7 , accompanied by the formation of sharply angular nano-sized crystallites (about 50 nm). No crystalline phases other than (VO) 2 P 2 O 7 were formed during this transformation. This is quite different from the transformation of large and thick VOHPO 4 ⋅0.5H 2 O crystallites, in which α II -VOPO 4 and δ-VOPO 4 have been detected in the resulting catalyst. The selectivity to maleic anhydride increases during the transformation as a result of the formation of (VO) 2 P 2 O 7 . The activity increases in the early stages of the transformation, then decreases with time. The decrease in activity is caused by gradual reduction of the catalyst surface during the transformation.