Non-Faradaic (ion electrosorption) and Faradaic (oxidation–reduction) effects in a batch-mode capacitive deionization (CDI) system were investigated, with results showing that both effects were enhanced with an increase in charging voltage (0.5–1.5 V). Significant concentrations of hydrogen peroxide (H₂O₂) were observed with the generation of H₂O₂ initiated by cathodic reduction of O₂ with subsequent consumption occurring as a result of cathodic reduction of H₂O₂. A kinetic model of the Faradaic processes was developed and found to satisfactorily describe the variation in the steady-state concentration of H₂O₂ generated over a range of CDI operating conditions. Significant pH fluctuations were observed at higher charging voltages. While the occurrence of Faradaic reactions may well contribute to pH fluctuations and deterioration of electrode stability and performance, the presence of H₂O₂ could provide the means of inducing disinfection or trace contaminant degradation provided H₂O₂ could be effectively activated to more powerful oxidants (by, for example, ultraviolet irradiation).