The aqueous oxidation of S(IV) by H2O2 is one of the principal paths of acid formation in the atmosphere. Several investigators have measured the rate constant for the reaction: H2O2 (a)+S(IV)(a)→H2SO4(a), as a function of temperature and pH. The reactant concentrations used, however, were several orders of magnitude greater than those found in precipitation or cloud particles. We have measured the rate constant at more typical environmental concentrations by using a different analytical technique in which excess S(IV) (10-6 to 10-5M) was reacted with H2O2(~10-7M). We followed the decay in H2O2 concentration with time by using an H2O2 luminol-chemiluminescent instrument. The lower reactant concentrations we use obviate the use of buffers at our working pH range of 4.0 to 5.8. Our measurements of the S(IV)-H2O2 rate constant were in substantial agreement with the previous measurements. We noted a first order dependence in the rate constant with hydrogen ion concentration and at 22 ¿C find our results can be represented by k=(8.03¿0.18)¿107+> dm3 mole-1 s-1 when the pH is between 4.0 and 5.8.