A new photolabile protecting group (PPG) for carboxylic acids and amines has been developed based on the rapid singlet state photodecarboxylation of xanthone acetic acids with several features that are superior to many other systems. We demonstrate that the "xanthonate" PPG can photorelease carboxylic acids and amines (via the carbamates) quantitatively in neutral phosphate buffer solution with a remarkable "uncaging cross section" (Phi . epsilon = 3900 M-1 cm(-1) with UVA irradiation). Advantageous features include a high reaction efficiency (Phi > 0.6), very clean photochemistry, subnanosecond release kinetics, good water solubility, and excellent UVA absorption. Investigations into the mechanism of release employing nanosecond laser flash photolysis (LFP), time-resolved and steady-state fluorescence spectroscopy, and product studies support an initial rapid decarboxylation from the singlet excited state to form a carbanion intermediate which undergoes a very rapid beta-elimination of the carboxylate or carbamate leaving group, all occurring on the subnanosecond time scale.