Evaluation that will predict with a high degree of accuracy the suitability for service of any product is the object of all physical testing. Over a comparatively short period of about 25 years, the rubber technologists have evolved a series of tests which have reasonably well evaluated natural rubber compounds. When it became necessary to evaluate rubber substitutes, it was only natural that the same tests would be applied. The results obtained convinced everyone that these materials definitely were not equivalent substitutes and to use them involved more than just replacement. Although the background is not as extensive as might be desired, the A.S.T.M. Committee D-11 on Rubber Products has learned enough about these materials to be able to present certain facts and recommendations to industry in general with regard to specifications covering synthetic rubbers and products made from them.