The measurements of the permeability of various elastomers to hydrogen which have been described make particularly evident the following points. The permeability values of Perbunan, Neoprene and Oppanol are from one-third to one-eighth those of natural rubber. The higher the percentage of combined sulfur in natural rubber, the lower the permeability. The addition of a plasticizing agent has only a small effect on the permeability. The addition of progressively greater percentages of inorganic or organic fillers brings about a disproportionately great reduction in permeability. Among mixtures of natural rubber with other compounding ingredients, three mixtures in particular are noteworthy for their low permeability: (1) A mixture of equal parts of natural rubber and Oppanol (polyisobutylene). (2) A mixture of rubber with 15–20 per cent by vol. of bentonite, prepared by starting with latex. (3) A combination of these two types of mixtures (1) and (2). Finally it was proved experimentally that stretched rubber has a lower permeability than the corresponding unstretched rubber, probably as a result of crystallization of the stretched rubber.