Granular activated carbon (GAC) can be used to quench residual hydrogen peroxide (H2O2) after ultraviolet (UV)-H2O2 advanced oxidation processes in drinking water treatment. Bench- and pilot-scale studies examined the possible loss of GAC reactivity due to continuous exposure to natural water from a water treatment plant in both the presence and absence of H2O2. Six common types of GAC were evaluated. The bench- and pilot-scale tests consistently showed that all GACs lost a significant degree of reactivity with H2O2 during the first 25,000 bed volumes of water treated, but then further deterioration to beyond 100,000 bed volumes was minimal. The loss of reactivity was due mostly to exposure to the natural water, with continuous exposure to H2O2 having little impact, suggesting that the reaction with H2O2 may be catalytic whereas the GAC becomes fouled by natural organic matter or other constituents in the natural water. The six GACs exhibited different reaction rates with H2O2, suggesting that GAC type might be an important consideration for process design.