The development of a free molecule probe for use as an instrument in the investigation of two dimensional rarefied gas flow fields is described. The probe consists of a cross‐stream cylindrical wire of diameter small compared with the mean free path of the gas. Measurement of the probe temperature and heat transfer characteristics yields information that can be related theoretically to the state of the flowing gas. This paper describes the use of such information in the study of the qualitative nature of supersonic flow about sharp leading edges in regions where rarefaction, slip, and boundary layer‐shock wave interaction effects are important. The results showed clearly the effect of increasing density. Thus, at the lowest densities and Mach numbers, a clearly defined shock wave and boundary layer emerged from a region of mixed compressive and viscous action at the leading edge, while at the higher densities and Mach numbers there was a large region of strong boundary layer‐shock wave interaction and a considerable delay in the formation of clearly defined shock wave.