A model suitable for studies of non-equilibrium effects in chemical reactions (13) was used to predict incubation times for the dissociation of O2 in Ar between 2900 and 7200 °K. It was found that the model accounts satisfactorily for the experimental data. On the other hand previously used theories underestimate these incubation times by a factor of five to ten; the discrepancy is traced to assumptions used in previous investigations. When the same model is applied to the dissociation of Br2 in Ar and Cl2 in Ar, it correlates well a variety of experimental data, obtained over a wide temperature range.