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Measurements-of shock front thicknesses in argon by the reflectivity method have been extended to Mach 2.09, and the shape of the front has been investigated by comparing the experimental results with the reflectivity expected on the basis of five simple models for the density change through the front. The thicknesses agree with Zoller's calculations but for the strongest shocks are considerably greater than those of other ~eoretical.estim~tes. The magnitude and form of the reflectivity (as a function of XIL cos8) of shock fronts In three diatomIc gases have been used to study the rate of equilibration of rotational with translational energy. Hydrogen requires more than one hundred and fifty collisions for this equilibration while nitrogen and oxygen equilibrate much more rapidly. For the latter two gases there is evidence that at least two relaxation times are involved in the equilibration. 9 A.