Abstract In the manufacture of rubber goods various materials are incorporated with the rubber in order to produce a mixture that will process more easily. Such materials are usually called fluxes or softeners. A simple method of determining the relative effects of such materials is described. This consists in mixing the material in rubber under standardized conditions and determining the plasticity of the product by the Williams method. Some materials show stiffening instead of softening action and both types are shown in the results reported in this paper. Benzidine and p-amidophenol are examples of materials which stiffen rubber. MANY materials used in the compounding of rubber exert an influence on the plasticity. Usually a material is specifically added to increase the plasticity and is called a softener, but in some cases the effect on plasticity is purely incidental to some more important function, such as acceleration or protection against deterioration on aging. The effect of this class of materials may be towards either a more or a less plastic mix; in the latter case it is correct to class the material as an antisoftener.