An experimental study of the diffusion of helium in the wake of a circular cylinder was conducted in the GALCIT hypersonic wind tunnel at a Mach number of 5.8. The cylinder was constructed of material having random porosity and was mounted with its axis perpendicular to the stream. The light gas was injected in small amounts and the thermal conductivity method was utilized to detect the concentration of helium in the air at points downstream. Problems in the utilization of the thermal conductivity method for low sample densities were overcome by suitable calibration. Flow in the wake of the cylinder was found to display characteristically similar behavior at a few diameters downstream, with respect to decay and spread of the concentration. Reynolds number similarity was established in the laminar case, but turbulent Reynolds number similarity may require reference to momentum thickness, which was not possible with the present data. Profile data was somewhat marred by a tunnel pressure perturbation, but many of the important conclusions were not affected. The profiles appear to follow the theoretical Gaussian distribution in the similar region. The thermal conductivity method is quite promising as a means of tracing the diffusion of one binary gas constituent in another, as applied to hypersonic wind tunnel experiment. It will also serve in the analysis of transition and turbulence, and of the lateral spreading of the turbulent fluid into the rest of the wake region behind the bow shock.