Abstract The present classification program for natural rubbers has been handicapped by the lack of adequate means to characterize their behavior on vulcanization. Several methods have recently been proposed for this purpose. Roth and Stiehler described the strain test from which three vulcanization parameters can be calculated. Two of these parameters have been combined by Schade. Thirion proposed a method for calculating two parameters from the customary measurements of stress at 600 per cent elongation. Gee and Morrell have recently suggested a procedure for determining two parameters from measurements of Mooney viscosity and stress at 100 per cent elongation. Workers at The B. F. Goodrich Company have proposed the use of the Mooney viscometer for determining two parameters. The present study was initiated to compare these methods for classifying natural rubbers. At the same time, an investigation was made of several conditions of treatment of the compound before vulcanization and of the temperature of vulcanization to determine their relative effects on the parameters and the classification of the rubbers. For this work, four Hevea rubbers that covered a wide range in both rate of cure and ultimate stiffness of the vulcanizate were chosen. The rubbers were smoked sheet, pale crepe, sprayed latex, and Tensorub. The sample of smoked sheet was from a bale of No. 1 grade rubber, taken at random. The same was true for pale crepe. The sample of sprayed latex rubber was prepared by the U. S. Rubber Co. plantation by spray-drying once-creamed formaldehyde-preserved latex. The sample of Tensorub was produced by Socfin Co., Ltd., in Malaya and was selected to represent a high-quality smoked sheet.