In situ-FTIR, in situ-UV-vis and operando-EPR spectroscopic methods were used to study the interaction of 1-butene/N-2, 1-butene/O-2/N-2 and 1-butene/O-2/N-2/H2O mixtures with-VOx/TiO2 (6.1 wt.% V) and VOx/SbOy/TiO2 (4.2 wt.% V, 9.5 wt.% Sb) catalysts under reaction-like conditions. The catalysts were prepared by spray drying of oxide mixtures (V2O5/anatase and V2O5/Sb2O3/anatase) and calcination in air at 400degreesC. Butene reacts already at room temperature intensively with V surface sites indicated by a strong increase of the VO2+ signal intensity during EPR measurements and the appearance of adsorbed oxidized products (ketone, enolate) detected by FTIR. UV-vis experiments revealed a similar reduction-reoxidation behavior of the two catalysts characterized by a fast process assigned to V sites on the surface followed by a slower one attributed to V sites in deeper layers of the catalyst structure. During reaction at 200degreesC, the VO2+ EPR signal intensity increases continuously for the VOx/TiO2 catalyst. For the Sb-containing VOx/TiO2 catalyst the formation of V3+ seems to be favored. A rapid reaction accompanied by the formation of carboxylate, mainly acetate, and cyclic anhydride has been observed by FTIR investigations. Whereas the two catalysts show in principle a similar behavior, the amount of adsorbates was much lower on the Sb containing VOx/TiO2 catalyst which indicates a higher activity of the Sb-free catalyst. The admixture of Sb modifies the acidity of the catalyst and changes the redox behavior of the active VOx species. In catalytic tests the Sb-containing VOx/TiO2 catalyst acts more selectively. By adding water vapor to the feed, the formation of anhydride being a precursor for total oxidation is markedly suppressed and carboxylate is preferably formed. (C) 2004 Elsevier B.V. All rights reserved.