Chlorine dioxide (ClO₂) decay in the presence of typical metal oxides occurring in distribution systems was investigated. Metal oxides generally enhanced ClO₂ decay in a second-order process via three pathways: (1) catalytic disproportionation with equimolar formation of chlorite and chlorate, (2) reaction to chlorite and oxygen, and (3) oxidation of a metal in a reduced form (e.g., cuprous oxide) to a higher oxidation state. Cupric oxide (CuO) and nickel oxide (NiO) showed significantly stronger abilities than goethite (α-FeOOH) to catalyze the ClO₂ disproportionation (pathway 1), which predominated at higher initial ClO₂ concentrations (56–81 μM). At lower initial ClO₂ concentrations (13–31 μM), pathway 2 also contributed. The CuO-enhanced ClO₂ decay is a base-assisted reaction with a third-order rate constant of 1.5 × 10⁶ M^–2 s^–1 in the presence of 0.1 g L^–1 CuO at 21 ± 1 °C, which is 4–5 orders of magnitude higher than in the absence of CuO. The presence of natural organic matter (NOM) significantly enhanced the formation of chlorite and decreased the ClO₂ disproportionation in the CuO–ClO₂ system, probably because of a higher reactivity of CuO-activated ClO₂ with NOM. Furthermore, a kinetic model was developed to simulate CuO-enhanced ClO₂ decay at various pH values. Model simulations that agree well with the experimental data include a pre-equilibrium step with the rapid formation of a complex, namely, CuO-activated Cl₂O₄. The reaction of this complex with OH^– is the rate-limiting and pH-dependent step for the overall reaction, producing chlorite and an intermediate that further forms chlorate and oxygen in parallel. These novel findings suggest that the possible ClO₂ loss and the formation of chlorite/chlorate should be carefully considered in drinking water distribution systems containing copper pipes.