K-doped Mo-V-Sb mixed oxides catalysts have been prepared by impregnation of a MoVSbO mixed oxide (previously prepared by hydrothermal synthesis) with an aqueous solution of potassium nitrate, characterized by XRD, SEM-EDX, EPR, XPS, XANES, and FTIR of adsorbed NH3, and tested in the selective oxidation of propane and propylene in the 593-693 K temperature range. The undoped MoVSbO catalysts presented a selectivity to acrylic acid lower than 15%, while selectivities to acrylic acid of about 40% can be obtained on K-doped catalysts. No appreciable differences between undoped and K-doped catalysts are observed in the nature of crystalline phases present in each case, on the basis of X-ray diffraction analysis. However, important differences in both the oxidation state of surface Sb species (according to XPS evidence) and the number of acid sites (determined from FTIR of adsorbed NH3) are observed between the undoped and the K-doped catalysts. The high selectivity to acrylic acid on the K-doped catalysts seems to be related to changes in the acid-base properties of the catalysts and most particularly to the elimination of (Bronsted) acid sites. In addition, the role of antimony species in activity and selectivity is also discussed and a reaction network for the partial oxidation reaction is proposed. (C) 2004 Elsevier Inc. All rights reserved.