p-Hydroxyphenacyl, a new photoactive, aqueous soluble protecting group is proposed as a second generation α- keto “cage” reagent, a phototrigger for the efficient, rapid release of bioactive phosphates, e.g., inorganic phosphate (Pi) and ATP (Givens, R. S.; Park, C.-H. Tetrahedron Lett. 1996, 37, 6259−6262). p-Hydroxyphenacyl esters 6c and 7 trigger the release of Pi and ATP when irradiated at wavelengths between 300−350 nm also yielding p-hydroxyphenylacetic acid (8) from the rearrangement of the intermediate α-keto carbocation or its equivalent. In contrast, unsubstituted and m-substituted phenacyl esters yield only photoreduction and radical coupling products and none of the rearrangement product. Quantum efficiencies of 0.38 ± 0.04 were measured for the disappearance of the p-hydroxyphenacyl phosphate esters 6c and 7; the appearance efficiencies for 8 and ATP were 0.30 ± 0.03. Rates of release of ∼107 s-1 or better are observed for these esters with only minor variations in efficiencies and rate constants between these two examples of the p-hydroxyphenacyl phototrigger. Just as was found for the desyl “cage” series reported earlier (Givens, R. S.; Athey, P. S.; Kueper, L. W., III; Matuszewski, B.; Xue, J.-y.; Fister, T. J. Am. Chem. Soc. 1993, 115, 6001−6010), the p-hydroxyphenacyl derivatives react via their triplet states. Amino substituents, i.e., p-amino-, p-acetamido-, and p-(carbomethoxyamino)phenacyl phosphates 6f−h, were also investigated, but these analogues proved to be inferior as phototriggers when compared with p-hydroxyphenacyl.