Photolysis of 4-hydroxyphenacyl (pHP) esters has been successfully employed for the release of carboxylic and amino acids for biological studies. These leaving groups have low pKa's and are representative of an increasing array of substrates amenable to this new technology. With the 4-YC6H4O- positioned to the ketone as the leaving group, the efficacy and efficiency of the pHP phenolate displays two divergent pathways for the photorelease. The mechanism followed is dependent on the nature of the Y substituent on the phenol. Quantum efficiencies, yields and effects of solvent, buffer, and pH will be discussed.