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An experimental investigation of the molecular separation effect in a Pitottype sampling probe aligned with a supersonic gas mixture flow is described in this paper. The investigation was intended primarily to determine a (generally applicable empirical) correlation of the probe separating effect over a wide range of flow conditions encompassing the region of maximum separation effect. For a better understanding of molecular flow, a numerical computation employing the direct simulation (Monte Carlo) method was made for a typical probe entry. This revealed the presence of radial diffusion currents in the stagnation zone created by strong temperature and pressure gradients. The diffusive nature of molecular separation thus observed is successfully described in the resulting correlation which groups the probe separation effect for a variety of gas mixtures in the mean molecular weight range (7.2-187) to a root-mean square error of 10%. The maximum separation factor obtained was found to be approximately 2.7 times the separation factor calculated from the square root of the molecular weight ratio.