The carbon dioxide/bicarbonate (CO2/HCO3-) molecular pair is ubiquitous in mammalian cells and tissues, mainly as a result of oxidative decarboxylation reactions that occur during intermediary metabolism. CO2 is in rapid equilibrium with HCO3-via the hydration reaction catalyzed by carbonic anhydrases. Far from being an inert compound in redox biology, CO2 enhances or redirects the reactivity of peroxides, modulating the velocity, extent, and type of one- and two-electron oxidation reactions mediated by hydrogen peroxide (H2O2) and peroxynitrite (ONOO-/ONOOH). Herein, we review the biochemical mechanisms by which CO2 engages in peroxide-dependent reactions, free radical production, redox signaling, and oxidative damage. First, we cover the metabolic formation of CO2 and its connection to peroxide formation and decomposition. Next, the reaction mechanisms, kinetics, and processes by which the CO2/peroxide interplay modulates mammalian cell redox biology are scrutinized in-depth. Importantly, CO2 also regulates gene expression related to redox and nitric oxide metabolism and as such influences oxidative and inflammatory processes. Accumulated biochemical evidence in vitro, in cellula, and in vivo unambiguously show that the CO2 and peroxide metabolic pathways are intertwined and together participate in key redox events in mammalian cells.