The kinetics of oxidation of colloidal chalcopyrite (CuFeS₂) have been examined in the presence of aqueous Fe^III and Cu^II at pH 2. 3 and in the presence of radiolytically generated Fe(OH)²⁺ at neutral pH. The reaction of Cu^II with CuFeS₂ has been ascribed to an ion-exchange reaction of copper for iron in the CuFeS₂ lattice. The initial copper sulfide phase that forms, as a result of the exchange, is rich in Cu^I and thermally converts to the known covellite (CuS) phase. Oxidation of CuFeS₂ by Fe^III at low pH leads to the dissolution of the chalcopyrite phase and the formation of Fe²⁺(aq), Cu²⁺(aq) and S°(s). The formation of an unstable intermediate CuS_y phase is suggested from thermodynamic calculations. Under neutral pH conditions, CuFeS₂ is oxidized by Fe(OH)²⁺ adsorbed on the particle surface. Oxidation is restricted to the first monolayer of the particles due to the formation of an Fe^III hydroxide layer at the particle surface. The oxidation of CuFeS₂ by Fe(OH)²⁺ is slower than by Fe³⁺(aq), presumably due to the lower redox potential of the former species.