Abstract: Methods and apparatus for determining the permeation behavior of foreign fluid molecules into solids whereby the results are typically obtained in less than one day. In a principal embodiment, information about the relationship between the saturation concentration and the diffusion constant of the solid is obtained by subjecting a first, thick, sample of the solid to the foreign fluid substance, which has been radioactively-tagged with a class C isotope, for a short diffusion time interval. The penetrated foreign substance is then washed out of the thick sample with an untagged solvent and the resulting solution is then analyzed to determine the quantity of the foreign substance that has diffused into the thick sample for the diffusion time. A second, thin, sample of the solid is prepared, and analyzed in a series of steps analogous to the thick, sample, whereby information concerning the saturation concentration, individually, is obtained.