This article reports a study of excited-state hydroxide ion release from a model xanthenol photobase, XanOH. The driving force for the reaction was tuned using solvent mixtures with varying water/acetonitrile ratios, and the kinetics of the reaction was monitored using ultrafast pumpprobe spectroscopy. The intrinsic barrier for the heterolysis was evaluated using Marcus and bond-energy bond-order (BEBO) models. The obtained value (Delta G degrees# = 10.1710.80 kcal/mol) is significantly higher than the intrinsic barriers found for the proton release from previously studied photoacids. These results were discussed in terms of the difference in structures of solvated H+ and OH- ions.