The dehydrogenation of the mono-, di-, and trimethyl-substituted benzenes (toluene, o-, m-, p-xylene, and mesitylene) on niobium cluster cations Nb-n(+), n = 2,..., 26, in the gas phase was investigated in an FT-ICR mass spectrometer. While most Nb-n(+) clusters dehydrogenate the compounds completely, a partial dehydrogenation is observed for very small clusters with n less than or equal to 6 and clusters in the range 15 less than or equal to n less than or equal to 19. Selectively deuterated xylenes are used to clarify the mechanisms of the isomer specific dehydrogenation reactions, and the observed results and trends are discussed.