The solubility constants for thirteen permanent gases were measured within the temperature range 5–55°C. in a linear and a branched polyethylene, hydrogenated polybutadiene, and natural rubber. Equilibrium and time lag solubility determinations were made, the latter being indirectly obtained as the ratio of the permeability to the diffusion constants. Within experimental precision, equilibrium and time lag measurements were in agreement. The solubility of each gas in polyethylene was found to be proportional to the volume fraction of amorphous material when the polymer was treated as a simple, two-phase mixture of crystalline and amorphous polymer, each with a characteristic specific volume. Even the small helium molecule exhibited no detectible solubility in the crystallites according to this model. Therefore, a solubility constant in completely amorphous polyethylene could be determined for each gas which, with the density of an arbitrary sample, would enable calculation of the solubility of each gas in that particular sample. A correlation of the solubility constants in completely amorphous polyethylene was obtained from a thermodynamic model of the disolution process. Comparison of this correlation with a similar one for normal hydrocarbon liquids emphasized the analogy between the amorphous phase in polyethylene and these low molecular weight substances. The solubility constants in natural rubber, a completely amorphous polymer under the experimental conditions, were approximately 50% higher than those in amorphous polyethylene. Higher intermolecular forces due to the unsaturation in natural rubber may account for this result. The apparent heats of solution of all gases in branched polyethylene were approximately 1.0 kcal./g. mole more positive than those in linear polyethylene. This behavior was shown quantitatively, by comparison with dilatometric data, to be simply the result of crystalline melt-out in branched polyethylene. In linear polyethylene the experimental solubility data yielded essentially true heats of solution, since negligible crystalline melt-out occurred between 5 and 55°C. These were correlated quite satisfactorily by further application of the thermodynamic model for the dissolution process. Available evidence, therefore, indicates that the crystallites in polyethylene are impenetrable, and are randomly distributed on a macroscopic scale with respect to the diffusion and dissolution processes. The amorphous phase behaves as a homogeneous liquid whose thermodynamic properties are independent of the mode of polymer preparation, thermal history of the sample, and level of crystallinity.