Abstract Electrochemical oscillations of a new type, named oscillations C and D, appear, in addition to previously reported oscillations, named oscillations A and B, in the H 2O 2-reduction reaction on a Pt electrode in acidic solutions when small amounts of halide ions such as Cl −, Br −, and I − are added to the solution. Oscillation A appears in a narrow potential region of −0.25 to −0.32 V versus SCE where adsorbed hydrogen (upd-H) of a nearly full coverage is formed, and oscillation B appears in a region below ca. −0.35 V where hydrogen evolution occurs, whereas oscillations C and D appear in potential regions more positive than −0.30 V where halide ions are adsorbed on Pt. The detailed behavior of oscillations C and D is described and compared with that of oscillations A and B. Mathematical simulation has shown that the essential features of oscillations C (and D) can be understood by a reaction model which includes a kind of autocatalytic mechanism in which adsorbed halogen and OH promote dissociative adsorption of H 2O 2.