The influence of vanadium content on the performance of V-Mg-O catalysts for the oxidative dehydrogenation (ODH) of propane was investigated. High-surface-area (380 m(2)/g) MgO was prepared by hydrolysis of Mg(OCH3)(2) followed by hypercritical drying. Vanadia was deposited on this support by thermolysis of OV((OBu)-Bu-t)(3). Catalysts prepared by this means have BET surface areas of 187-299 m(2)/g and apparent surface densities of V2O5 of 1.1-10.3 VOx/nm(2). All of the catalysts were characterized by X-ray diffraction, temperature-programmed reduction, and Raman, UV-visible, and nuclear magnetic resonance spectroscopy. The environment of the V atoms depends strongly on the apparent surface density of vanadia. Isolated VO42- units are present at very low apparent surface densities (similar to 1 VOx/nm(2)). As the vanadia density increases, magnesium vanadate structures are formed and above a surface density of 3.5 VOx/nm(2) well-dispersed magnesium orthovanadate domains become evident. The rate of ODH per V atom increases with increasing VOx surface density and reaches a maximum value at 3.5 VOx/nm(2). Above this surface density, the rate of ODH per V atom decreases because an increasing fraction of the V atoms lie below the catalyst surface and, hence, are inaccessible. Consistent with this interpretation, the ODH activity per unit surface area reaches a plateau at a VOx surface density of about 4VO(x)/nm(2). The propane ODH selectivity of the catalysts increases with increasing VOx surface density and reaches a plateau of 80% for an apparent surface density of about 4VO(x)/nm(2). Rate coefficients for propane ODH (k(1)), propane combustion (k(2)) and propene combustion (k(3)) were calculated for each catalyst. The value of k(1) increases with increasing VOx surface density, reaching a maximum at about 4 VOx/nm(2). By contrast, the ratios (k(2)/k(1)) and (k(3)/k(1)) decrease monotonically with increasing VOx surface density. The observed trends in k(1), (k(2)/k(1)), and (k(3)/k(1)) are discussed in terms of the surface structure of the catalyst.