Vanadyl pyrophosphate catalysts prepared via VOHPO(4)(.)1.5H(2)O precursor were calcined in a reaction flow of n-butane(0.75%)/air mixture at 673 K for a duration of 10, 30 and 75 h. These catalysts were denoted as VPS10, VPS30 and VPS75. Increasing the duration of calcination process led to the appearance of V5+ phase (VOPO4), which could be seen in XRD patterns at 2theta = 29.1degrees and an increment of the average oxidation number of the vanadium. It also led to a decrease in the total surface area from 27.2 (VPS10) to 22.1 (VPS30) and 17.7 m(2) g(-1) (VPS75). Scanning electron microscope showed the secondary structure of the catalysts produced, consisting of plate-like crystals in different sizes, which were agglomerated into the characteristics of rosette-shape clusters. However, at closer and higher magnification show the surface of each platelet getting cracked and rougher as the pretreatment time increased. The nature of the oxidants in/on the catalysts was investigated by using O-2 TPD and TPR in H-2. VPS10 gave a broad peak having an onset at 830 K and a peak maximum at 970 K with a shoulder at 1005 K. An increment of the calcination duration to 30 h (VPS30) gave a peak at 974 K and the shoulder at 1005 K (VPS10) becomes a peak at 1006 K. This new phase peak was found to be more dominant at 1018 K as the calcination duration increased to 75 It (VPS75), whereas the first peak reduced to a shoulder at 980 K. The forming of a new peak may be due to the additional phase (i.e. V5+), which is shown in the XRD and chemical analysis. TPR profiles of all the catalysts gave three peaks. VPS10 gave the highest total amount of oxygen removed, i.e. 2.15 x 10(21) atom g(-1) compared to 1.94 x 10(21) and 1.72 x 10(21) atom g(-1) for VPS30 and VPS75, respectively. (C) 2004 Elsevier B.V. All rights reserved.