Density distributions were experimentally determined in rarefied gases at rest contained between two parallel plates maintained at widely different temperatures. The temperatures of the hot and cold plates were ∼79°K and ∼294°K, respectively, resulting in a temperature ratio of about 4:1. Both helium and nitrogen were used as test gases. Particular attention was focused on obtaining data in the temperature jump and transition regimes. The gas densities were measured by observing the luminescence produced by a high-energy electron beam traversed between the plates. In addition to the density measurements, heat transfer measurements were also made in order to estimate the values of the thermal accommodation coefficients. The experimental results for helium were compared with the analytical results of Liu and Lees, and good agreement was found between the theory and data near the cold plate. Near the hot plate the analytical results and the data differed by about 7%.