The kinetics of hydrogen peroxide formation is studied during the oxidation of styrene epoxide (SE) in the presence of para-toluenesulfonic acid (TSA) in a solution of 90 vol % tert-butanol with 10 vol % chlorobenzene. The hydrogen peroxide buildup rate (v(HP)) is proportional to the toluenesulfonic acid concentration and independent of the concentration of styrene epoxide: v(HP) = k(HP)[SE](0)[TSA](1) at [SE] much greater than [TSA], k(HP) = 2.9 x 10(-4) s(-1) (343 K). The rates equations for three processes, namely, oxygen uptake, benzaldehyde (BA) accumulation, and hydrogen peroxide accumulation, are identical. The concentration ratio of H2O2 formed to oxygen absorbed is constant: [H2O2]/[O-2](n) = 0.4. This behavior was interpreted in terms of the empirical reaction scheme SE + O-2 --> BA + 0.5 H2O2.