Advanced reduction processes (ARPs) that generate hydrated electrons (eaq –; 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 eaq – or shield incoming UV photons from the applied photosensitizer. This study examined the pH-dependent quenching of eaq – by ubiquitous dissolved carbonate species (H2CO3*, HCO3 –, and CO3 2–) and quantified the relative importance of carbonate species to other abundant quenching agents (e.g., H2O, H+, HSO3 –, and O2(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 eaq – quenching by H2CO3*, HCO3 –, and CO3 2– ( k H 2 C O 3 * = 2.23 ± 0.42 × 109 M–1 s–1, k H C O 3 − = 2.18 ± 0.73 × 106 M–1 s–1, and k C O 3 2 − = 1.05 ± 0.61 × 105 M–1 s–1), with quenching dominated by H2CO3* (which includes both CO2(aq) and H2CO3) at moderately alkaline pH conditions despite it being the minor species. Attempts to apply previously reported rate constants for eaq – quenching by CO2(aq), measured in acidic solutions equilibrated with CO2(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 eaq – quenching by varying H+ can instead be ascribed to variable acid–base speciation of dissolved carbonate and the sulfite sensitizer.