This is a picosecond and nanosecond time-resolved infrared absorption spectroscopic study of p-hydroxphenacyl diethylphosphate (HPDP) in aqueous solution examining the deprotection reaction and subsequent evolution to produce the p-hydroxyphenylacetic acid (HPAA) rearrangement product. The data reported here support previous ultrafast fluorescence, Raman and transient UV/visible absorption studies and provide new insight into the role of water in the deprotection reaction. It also provides insight into the nature of the intermediate steps that take place following initial formation of the triplet state and its reaction with water to eventually produce the solvent-separated ions and solvolytic reaction product HPAA. In addition, density functional theory (DFT) calculations were performed to investigate the reactions of the triplet state of HPDP with water molecules to induce deprotection, the formation of a contact ion pair and the subsequent intermediates that lead to the HPAA rearrangement product. A more detailed reaction mechanism is proposed based on the time-resolved spectroscopy experiments and DFT computational results. ; Department of Applied Biology and Chemical Technology