We investigate the gas phase proton transfer reactivity of the dianionic molybdenum oxysulfide clusters [Mo2O2S6]2- and [Mo2O2S5]2- in binary collisions in an FT-ICR mass spectrometer in combination with quantum chemical calculations. The proton transfer reactions are probed with a series of acids with increasing proton affinity, namely trifluoroacetic acid, difluoroacetic acid, pyruvic acid and formic acid. Proton transfer followed by Coulomb explosion is observed for both dianions with all acids except for formic acid. The calculations predict a proton affinity of 16.1(1) and 16.5(1) eV for [Mo2O2S6]2- and [Mo2O2S5]2-, respectively, significantly higher than the proton affinities of tri- and difluoroacetate as well as pyruvate and formate, which range from 14.00(18)-14.98(10) eV. Calculated reaction potential energy surfaces explain the observations. A Coulomb barrier separating the collision complex from the products is present in all cases. The barrier lies well below the entrance channel for trifluoroacetic acid, difluoroacetic acid and pyruvic acid. For formic acid, however, the barrier lies near the entrance channel.