Simple blending of natural rubber/ethylene-propylene-diene rubber (NR/EPDM) generally results ill interior mechanical properties because Of curative migration and their differences for filler affinity. Ill this Work, the 70/30 and 50/50 NR/EPDM blends prepared by reactive processing techniques were investigated and compared with file simple, nonreactive blends. The reactive blend compounds, were prepared by preheating EPDM, containing all curatives to a predetermined time related to their scorch time prior to blending with NR. For the 70/30 gum blends, four types of accelerators were studied: 2,2-mercaptobenzothiazole (MBT), 2,2-dithiobis(benzothiazole) (MBTS), N-cyclohexyl-2-benzothiazolesulfenamide (CBS), and N-tert-buty-2-benzothiazolesulfenamide (TBBS). When compared with the simple blends, the reactive blends cured with CBS and MBTS showed a clearly improved tensile strength whereas the increase of tensile strength in the blends cured with TBBS and MBT was marginal. However, a dramatic improvement Of Ultimate tensile properties in the reactive 50/50 NR/EPDM blends cured with TBBS was observed when compared with the simple blend. For the N-550-filled blends at the blend ratios of 70/30 and 50/50, the reactive-filled blends prepared Under file optimized preheating times demonstrated Superior tensile strength and elongation at break over the simple. blends. The improved crosslink and/or filler distribution between file two rubber phases in the reactive blends accounts for such improvement in their mechanical properties. This is shown in file scanning electron micrographs of the tensile fractured surfaces of the reactive blends, which indicate a more homogeneous blend. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2035-2043, 2009