The chemiluminescence originating from OH ∗ , the excited hydroxyl radical, is one of the most extensively used diagnostics to characterize auto-ignition delay time of gaseous mixtures behind reflected shock waves. We have carried out new experiments and modeling of this diagnostic as well as analyzed previous results for hydrogen-based mixtures, including H 2 –O 2 , H 2 O 2 –H 2 O, H 2 –N 2 O and H 2 –O 2 –N 2 O. The experiments were analyzed with a detailed chemical reaction model which included mechanisms for OH ∗ creation, quenching and emission. Simulations of the reaction behind reflected shock waves were used to predict OH ∗ emission profiles and compare this with measured results as well as profiles of temperature and the ground state concentrations of OH. Analysis of OH ∗ rates of progress demonstrates that a quasi-steady state approximation is applicable and an algebraic model for OH ∗ concentrations can be derived that relates emission to the product of concentrations of O and H for H 2 –O 2 and H 2 O 2 mixtures and an additional contribution by the product of H and N 2 O when N 2 O is an oxidizer.