The aniline point of hydrocarbon oils and solvents is a satisfactory index of the swelling of oil resistant synthetic rubber compositions. The logarithm of the percentage volume increase varies inversely with the aniline point up to 100 per cent swelling. The slope of the swelling curve is apparently characteristic of each synthetic rubber, and is not affected by loading, temperature or degree of cure. Slight swelling does not greatly decrease the tensile strength, but above 100 per cent swelling the strength is greatly reduced.