This study investigates the oxidative degradation of N-nitrosodimethylamine (NDMA), a probable human carcinogen, by conventional ozonation and the advanced oxidation process ozone/hydrogen peroxide (AOP O3/H2O2). The rate constants of reactions of NDMA with ozone and hydroxyl radical (•OH) were determined to be 0.052±0.0016 M⁻¹ s⁻¹ and (4.5±0.21)×10⁸ M⁻¹ s⁻¹, respectively. The experiments performed with buffered deionized water varying solution pH and employing H2O2 and HCO3⁻ clearly showed that the reaction with •OH dominates the NDMA oxidation during ozonation. Conventional ozonation with up to 160 μM (=7.7 mg L⁻¹) ozone led to less than 25% NDMA oxidation in natural waters. The AOP O3/H2O2 required 160–320 μM ozone ([O3]0/[H2O2]0=2:1) to achieve 50–75% NDMA oxidation. However, multiple injections of ozone of the same overall dose somewhat improved the oxidant utilization efficiency by minimizing •OH scavenging contribution of oxidants. Methylamine (MA) was found to be a major amino product from NDMA oxidation initiated by •OH. The mechanism of NDMA oxidation to MA is discussed based on the results obtained in this study and the previous literature. Bromate formation may be the limiting factor for NDMA oxidation during ozonation and ozone-based AOPs in bromide-containing waters.