An expression is developed for the relaxation time for the approach to equilibrium for the dissociation of a diatomic gas behind a shock wave. This expression shows the importance of the variation of temperature behind a shock on the rate of approach to equilibrium. Numerical calculations of the shock wave profile in 02for two different dissociation mechanisms show that the relaxation time analysis is only appropriate for the description of the dissociation within a few percent of equilibrium. The calculations also show that ozone is not an effective intermediate for the shock dissociation of 02and that it is impossible to define a relaxation time for the over‐all dissociation process which is inversely proportional to pressure or density.