The premature vulcanization of rubber materials is substantially inhibited by incorporating therein a sulfenamide of the formula: ##SPC1##Wherein R, which may be the same or different, is alkyl of 1 to 8 carbon atoms. The preferred sulfenamide is N-phenythio-3,5-lupetidine. Said sulfenamide is incorporated into the rubber with or without a conventional vulcanization accelerator and is preferably employed in the form of a powder preparation adsorbed in an inorganic carrier.