Mo-V-Te-Nb mixed oxide catalysts, prepared by a slurry method at various pHs (in the range of about 4.5-1.0), have been tested in the partial propane oxidation to acrylic acid. The best catalytic performances were achieved on catalysts prepared at a pH from 2.0 to 2.8. In this case, formation rates of acrylic acid per unit mass of catalyst and per unit time, STYAA, close to 50 g(AA) kg(cat) h(-1) have been obtained at 653 K. The characterization of the catalyst precursors (uncalcined samples) suggests the presence of (NH4)(6)TeMo6O24.nH(2)O in the sample prepared at a pH of 4.5. However, the crystallinity of the molybdotellurate decreases when the pH of the slurry decreases and amorphous materials are observed at pH of slurry lower than 3.0. In addition, the characterization results suggest that the pH of the slurry influences the partial incorporation of heteroatoms (V and Nb) and/or the presence of heteroatoms (V and Nb) highly dispersed on the (NH4)(6)TeMo6O24.nH(2)O in catalyst precursors, which determines the formation of active and selective crystalline phases. The characterization of heat-treated (873 K in a N-2 stream) samples suggests the presence of TeMo5O16 crystalline phase for the sample prepared at the highest pH. However, other crystalline phases, i.e. Te2M20O57 and Te0.33MO3.33 (M = Mo, V, Nb) and Mo5-x(V/Nb)(x)O-14, can be proposed in heat-treated samples obtained from precursors prepared at lower pHs. So, three crystalline phases could be involved in the selective oxidation of propane to acrylic acid. (C) 2003 Elsevier B.V. All rights reserved.