Methylrhenium trioxide, CH3ReO3, catalyzes the hydrogen peroxide oxidation of chloride ions in acidic aqueous solutions. The active catalytic species are the monoperoxo complex, CH3ReO2(eta(2)-O-2), A, and the diperoxo complex, CH3ReO(eta(2)-O-2)(2)(OH2), B. At 25 degrees C in 1.0 M HClO4 and mu = 2.0 M the rate constants for the reactions of A and B with Cl- were determined to be 0.059 +/- 0.007 and 0.124 +/- 0.005 L mol(-1) s(-1), respectively, and both were approximately 10(5) times faster than the rate constant for the uncatalyzed reaction under similar conditions. Kinetic experiments employed the initial-rate method; reactions were monitored by following the consumption of monochlorodimedone (MCD) at lambda = 270 nm. MCD reacts rapidly with the chloride ion oxidation product(s).