Selective oxidations of propane and propene to acrylic acid were investigated over hydrothermally prepared Mo-V-Te-O and Mo-V-Te-Nb-O mixed metal oxide catalysts. Mono-phasic crystalline materials were obtained for both catalysts with the same orthorhombic layered structure characterized by sharp peaks at 6.6, 7.9, 9.0, and 22.2degrees in XRD (Cu Kalpha) patterns, so that niobium was not necessary for creating the structure. However, the addition of niobium resulted in a clear change in catalyst morphology. Long rod-shaped crystals (30-50 mum) in the case of the Mo-V-Te-O system were observed by SEM, whereas aggregates of small cylinder-shaped crystals (300-500 nm length) in the case of the Mo-V-Te-Nb-O system. Both catalysts were highly active for the oxidations of propane and propene and both showed almost the same catalytic performance in terms of substrate conversions. The results indicate that niobium was not influential in the oxidative activation of propane and propene. On the other hand, the selectivity to acrylic acid in both reactions increased by about 15-20% in the niobium-containing catalyst. This effect was much clearer at higher conversion. We discuss the formation of active orthorhombic structure during the hydrothermal synthesis and the role of niobium on the structure formation and on the catalytic performance of Mo-V-Te-O. (C) 2003 Elsevier B.V. All rights reserved.