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 H2O2-reduction reaction on a PI 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 H2O2.