AbstractElectrochemical potentials of photocatalysts are solvent dependent. One of the largest discrepancies is observed when water is used in place of organic solvents as the reaction media. Unfortunately, the redox potentials for many photocatalysts in water have not been determined, at least under one unifying set of conditions, and this greatly hinders the rational design of sustainable and biocompatible photoredox reactions. Herein, we measure the spectral and electrochemical properties of the most common photoredox catalysts in water and catalog their absorption and fluorescence maxima and ground- and excited-state potentials.