Mass spectrometric experiments are used to examine the size-dependent interactions of bare vanadium cluster cations V-n(+) (n = 1-7) with methanol. The reactivity patterns exhibit enormous size effects throughout the range Of Clusters investigated. For example, dehydrogenation of methanol to produce VnOC+ is only brought about by clusters with n >= 3. Atomic vanadium cation V+ also is reactive, but instead of dehydrogenation of the alcohol, expulsions of either methane or a methyl radical take place. In marked contrast. the reaction efficiency of the dinuclear cluster V-2(+) is extremely low. For the cluster cations V-n(+) (n = 3-7), complete and efficient dehydrogenation of methanol to produce VnOC+ and two hydrogen molecules prevails. DFT calculations shed light oil the mechanism of the dehydrogenation of methanol by the smallest reactive Cluster cation V-3(+) and propose the Occurrence of chemisorption concomitant with C-O bond cleavage rather than adsorption of an intact carbon monoxide molecule by the cluster.