Accurate measurements of the density distribution in Ar and N2 shock waves have been made in a shock tube for the Mach number range from 1.55 to 9 and 10 respectively by the absorption of an electron beam. A modified absorption law has been used for data reduction. The density profiles were corrected for the influence of shock curvature and density rise behind the shock wave. The measurements in Ar agree to within 1% with those of Schmidt (1969) in the mean range of Ms but give a slightly smaller density gradient for Ms = 9. Comparison with various theories shows very good agreement with Bird's Monte Carlo simulation in the whole Mach number range for a simple repulsive intermolecular force law. Further, the agreement with the Mott-Smith density profile for the same interaction law is also good, and surprisingly is found to be better for lower than for higher Mach numbers. Qualitative agreement is obtained with the solutions of Hicks & Yen for hard-sphere and Maxwell molecules. The Navier-Stokes and BGK solutions are found to differ significantly from the present experiments even for the lowest measured Mach number (1·55), whereas the Burnett equation gives better agreement, especially with respect to the asymmetry of the profiles.The measured N2 profiles agree on the whole with the shock-tube measurements of other investigators but show substantial deviations from the low density wind-tunnel experiments of Robben & Talbot (1966b) for higher Mach numbers. Bird's ‘energy sink model’ (1971) is in agreement with the measured density profiles for a realistic interaction law and a suitable rotational collision number. Rotational relaxation in nitrogen is found to be very fast for all Mach numbers. Consequently the coupling between rotational and translational relaxation is very strong.

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