The catalytic oxidation of CO by Cu-exchanged high-silica zeolites (e.g., ZSM-5) has been investigated theoretically using density functional theory. Calculations reveal two distinct, parallel pathways for oxidation of CO: (i) adsorption of O-2 on a reduced Cu site followed by O atom abstraction by CO, and (ii) adsorption of CO followed by its reaction with O-2 to form a cyclic compound which decomposes to form CO2. The reduced site is regenerated via two different pathways, both of which involve oxidation of one or more CO molecules: (i) abstraction of atomic oxygen by CO from the oxidized active site, and (ii) formation of a carbonate species followed by its reaction with a molecule of CO. The relevance of these reactions to the reduction of NO is discussed.