Advanced reduction processes (ARPs) that generate hydrated electrons (e aq − ; e.g., UV-sulfite) have emerged as a promising remediation technology for recalcitrant water contaminants, including per-and polyfluoroalkyl substances (PFASs). The effectiveness of ARPs in different natural water matrices is determined, in large part, by the presence of non-target water constituents that act to quench e aq − or shield incoming UV photons from the applied photosensitizer. This study examined the pHdependent quenching of e aq − by ubiquitous dissolved carbonate species (H 2 CO 3 *, HCO 3 − , and CO 3 2−) and quantified the relative importance of carbonate species to other abundant quenching agents (e.g., H 2 O, H + , HSO 3 − , and O 2(aq) ) during ARP applications. Analysis of laser flash photolysis kinetic data in relation to pH-dependent carbonate acid−base speciation yields species-specific bimolecular rate constants for e aq − quenching by H 2 CO 3 *, HCO 3 − , and CO 3 2− (k H CO 2 3 * = 2.23 ± 0.42 × 10 9 M −1 s −1 , k HCO 3 = 2.18 ± 0.73 × 10 6 M −1 s −1 , and k CO 3 2 = 1.05 ± 0.61 × 10 5 M −1 s −1 ), with quenching dominated by H 2 CO 3 * (which includes both CO 2(aq) and H 2 CO 3 ) at moderately alkaline pH conditions despite it being the minor species. Attempts to apply previously reported rate constants for e aq − quenching by CO 2(aq) , measured in acidic solutions equilibrated with CO 2(g) , overpredict quenching observed in this study at higher pH conditions typical of ARP applications. Moreover, kinetic simulations reveal that pH-dependent trends reported for UV-sulfite ARPs that have often been attributed to e aq − quenching by varying [H + ] can instead be ascribed to variable acid−base speciation of dissolved carbonate and the sulfite sensitizer.