Abstract This paper presents an exploratory study of the refractive index of rubber. Previous observations on the refractive index of rubber have been, for the most part, restricted to transparent samples. In the present investigation, however, by using reflected light rather than light at grazing incidence, it was found possible to extend the range of measurements to rubber samples that were somewhat dark in color or that were very nearly opaque. Such being the case, determinations of refractive index were made not only on translucent samples of unvulcanized or soft vulcanized rubber, but also on binary mixtures of rubber and different compounding materials, and on compounds of rubber and sulfur containing 0 to 19 per cent of sulfur and covering the range from soft to semi-hard rubber. Different samples of unvulcanized Hevea rubber, of plantation origin, all showed approximately the same refractive index, irrespective of considerable variations in the nonhydrocarbon components. The index was not appreciably altered by the amount of mastication which rubber ordinarily receives in processing and mixing with compounding ingredients. Mixtures of rubber with substances insoluble in it showed the same index as the rubber itself, while mixtures with soluble substances differed in index from rubber by an amount which depended upon the index of the substance and the amount in solution. Measurements of refractive index, therefore, afford a means of measuring the solubility of substances in rubber and were employed for determinations of the solubility of sulfur and of a common antioxidant, phenyl-β-naphthylamine. Sulfur in combination with rubber had a relatively greater effect on the refractive index than did sulfur in solution. For vulcanized rubber-sulfur compounds in the soft-rubber range, the refractive index was a simple linear function of the percentage of sulfur and also of the temperature. Preliminary measurements on vulcanized compounds in the hard-rubber range indicate that the change in physical properties of rubber-sulfur compounds from soft to hard was accompanied by a decrease in the slope of the curve relating refractive index to temperature.