Abstract The OH(2Σ+2-Π) chemiluminescence in the partial equilibrium region of the H2–O2 reaction in mixtures with H2/O2/Ar=2/2/96, P1=5, 10, 20 Torr was studied in 1250–2000 K incident shock waves. The comparison between the temperature dependence of the OH(2Σ+) emission and that of the computer simulation of the H2–O2 reaction yields the activation energy of the chemical excitation process, assuming the same bi-radical process as former investigators used. The evaluated OH(2Σ+) profile, based on a simulation in which the chemical excitation, radiative, and non-radiative processes are accounted for, depends on the pre-exponential factor of the chemical excitation process. The correlations between the temperature dependence of the evaluated OH(2Σ+) profile and that of the emission profile in the partial equilibrium region gave the pre-exponential factor of the chemical excitation process. Thus, we could estimate the rate constant for O+H+Ar=OH(2Σ+)+Ar to be 1.0×109×exp(−11000 cal⁄RT).