The mechanism of hydroxide release from photoexcited PBMeOH was probed by ultrafast transient absorption (TA) spectroscopy. Figure shows the transient absorption spectrum of PBMeOH after nanosecond laser excitation, using the reaction conditions of the steady-state photochemistry, namely 0.1 mM PBMeOH was dissolved in 2:1 MeCN:H2O solution. At early time delays (Δt < 30 ns), a positive TA spectrum is observed that resembles the steady-state PBMe + absorption spectrum shown in Figure (trace at t = 0 min) with underlying absorption features in the 400-500 nm wavelength region. These underlying absorption features disappear at later times (Δt > 300 ns) to furnish a TA spectrum that matches the absorption spectrum of PBMe + . The presence of an additional species in the 400-500 nm region is supported by time-resolved decay measurements (Figure , inset), which show that an early time species decays with a 81 ns lifetime to leave the longer-lived PBMe + species that persists to the maximum delay time of our detection (τ > 20 ms). Normalization and subtraction of the early (9 ns) and later (90 µs) transient spectra reveals the spectrum of the early time species that is shown in Figure . ). The decay time constant of 3.1 ns for PBMe + in neat MeCN is consistent with observed recombination rates of solvent-caged ion pairs in polar, organic solvents, thus suggesting that -OH remains confined to a solvent-pair in the absence of H2O. Taken together, the excited state dynamics of PBMeOH are consistent with a mechanistic scheme for photochemical hydroxide release summarized in Figure . Photoexcited PBMeOH* undergoes prompt bond homolysis to PBMe• and •OH and bond heterolysis to PBMe + and -OH within the envelope of laser excitation. The homolytic primary photoproducts recombine within 34 ns to return the system to PBMeOH ground state. In the case of the primary heterolytic photoproducts, a solventcaged [PBMe + …OH -] ion pair is produced. In neat MeCN, the ion pair recombines in 3 ns. Conversely, in the presence of H2O, cage escape of -OH is possible. Based on the PBMe + species, persisting to τ > 20 ms, the long lifetime of the caged-escaped -OH ion enables it to be captured by CO2 to generate the HCO3 -that is observed in steady-state photolysis experiments. We note, that the cage escape yield is reflected by the production of PBMe + at long time scales (t > 1 µs), as indicated by the τ2 kinetics decay trace shown in Figure . Actinometric TA measurements were determined from the TA spectra at t = 2.25 µs to yield the photochemical quantum yield (QYPC) of PBMe + formation (details provided in SI).