Abstract: Emerging prospects for the generation of usable energy from renewable sources including sunlight, water (H2O) and carbon dioxide (CO2) have increased in prominence and necessity. Precise understanding of the rate constants for proton transfer events that drive the chemistries of solar fuel and energy generation is necessary for reaction selectivities and efficiencies. One class of molecules that show promise toward control of local pH conditions that can dictate reaction selectivity or be used as molecular sensors to garner information about local pH are photoacids, reversible proton donors that show an increase in acidity (and decrease in the acid dissociation constant) upon light absorption. In this presentation, I present photoacids as dye sensitizers that use light to sense and vary the local concentration of protonic species. I demonstrate the sensing capabilities of photoacids for aqueous proton acceptors including water, hydroxide, (bi-)carbonate, acetate and formate using steady-state and nanosecond time-resolved photoluminescence spectroscopies.  Using nanosecond transient absorption spectroscopy combined with careful control over photon fluence, dye concentration, and dye pKa, and comparing an analytical model for reversible proton-transfer events with experimental data as a function of aqueous pH, I characterize photoacid ground-state regeneration toward control of local concentrations of protonic species and uniquely tune whether ground-state reprotonation occurs from either hydronium or water. The research presented aims to contribute to the fundamental photochemical knowledge of proton transfer reactions that serve as foundations for control over their reactivities.