Sorption of gases or vapors by polymers was studied as early as the 1930s [1]. However, it was not until the 1960s that there was considerable publication of such results in the literature. The early interest in vapor sorption by polymers stemmed perhaps from the observation that polymer/solvent solutions were highly nonideal by comparison with ideal solutions of lower molecular-weight substances. However, as synthetic polymers, particularly rubbers, became of increasing commercial and strategic importance during World War II, vapor sorption studies took on added significance. After World War II, synthetic polymers other than elastomers began to be made in quantity, the pace accelerating ever more rapidly in the 1960s and 1970s.