The effect of Cs modification in the selective oxidation of butane at 520-degrees-C was studied over SiO2-supported catalysts containing about 6 or 0.6 wt% vanadium atoms using a butane/oxygen/helium mixture of 4/8/88. Dehydrogenation products (unsaturated C4’S) and carbon oxides were the primary products. For the supported sample of high vanadium loading where a significant amount of vanadium existed as V2O5 crystallites under reaction conditions, Cs modification significantly increased the selectivity for dehydrogenation and decreased the extent of reduction of the vanadium oxide crystallites in the reaction mixture, but had only small effects on the activation energy or rate of reaction. Unsupported V2O5 catalysts modified by Cs at a Cs/V ratio of 0.023 also showed enhanced dehydrogenation selectivity, a lower degree of reduction at steady state, and a significantly different temperature-programmed reduction profile. Cs was found to be segregated on the surface of the crystallites. For the supported samples of low vanadium loading, Cs modification decreased the intensity of the Raman peak that was indicative of highly dispersed vanadia species and increased the activation energy of the reaction. These data were consistent with the picture that Cs modification enhanced the formation of vanadia clusters on the support.