1-phenyl 2,4-dithiobiuret (DTB-II) and 1,5-diphenyl 2,4-dithiobiuret (DTB-III) were studied as secondary accelerators along with N-cyclohexyl benzhiaazyl sulphenamide (CBS) in the sulphur vulcanization of natural rubber. These binary systems were found to be very effective in reducing the optimum vulcanization time. Out of the two dithiobiurets DTB-II was found to be more active indiating a nucleophilic mechanism in the vulcanization reactions under review. In each case, the optimum dosage of the secondary accelerator required was arrived at. Tensile strength, modulus, elongation at break, hardness, compression set, heat build-up, resilience etc. of the vulcanizates, were studied. Vulcanizates given by the binary accelerator systems gave satisfactory balance of properties. Chemical characterization of the vulcanizates as also carried out to correlate the physical properties with the type of chemical crosslinks formed.