Rapid scan Fourier transform infrared (FTIR) spectroscopy and time-resolved single wavelength infrared (IR) spectroscopy have been applied to study the mechanism of photochemical release of adenosine 5’-triphosphate (ATP) from its P-3-[1-(2-nitrophenyl)ethyl] ester (caged ATP). Bands arising from phosphate and non-phosphate vibrations characteristic of the aci-nitro anion intermediate and from free ATP and byproducts of the nitrophenylethyl group have been assigned using C-13, N-15, and O-18 isotopomers of caged ATP, Monitoring several of these bands using time-resolved single frequency LR spectroscopy confirms that release of ATP occurs in a single exponential process synchronous with the decay of the nci-nitro anion intermediate, Spectral characteristics of the reaction products arising from the 1-(2-nitrophenyl)ethyl group in the absence and presence of dithiothreitol (DTT) have been determined. The major final byproduct from photolysis conducted in the presence of DTT is 3-methylanthranil. The mechanism of formation of this compound from 2-nitrosoacetophenone has been investigated in detail by a combination of spectroscopic, kinetic, and chemical methods and reconciled with earlier data, The byproduct species likely to be present on the time scale of most biological experiments using caged compounds is 2-hydroxyl-aminoacetophenone as a mixture of ring-chain tautomers.