Abstract: A method and apparatus for testing the gas diffusion characteristics of material samples includes a sealed test chamber in which one or more test cells are placed. A material sample is mounted on one end of a substantially cylindrical test cell. The chamber is then charged to a predetermined pressure and concentration of a test gas, such as radon. Periodically, gas samples are drawn through the other end of the test cell for analysis of test gas concentration, which indicates the capability of the material sample to diffuse, or prevent diffusion of, the test gas. For each test, a desired pressure differential is maintained across the material sample, and also between the chamber interior and ambient.