Rubber goods usually require a combination of properties that cannot be provided by one elastomer only and then two or more polymer components have to be mixed to meet specific requirements. In such cases, the additives normally employed in rubber formulations are unevenly distributed, depending on the affinity of each compound to each polymeric phase. Thus, the dispersion of each one of these ingredients in the different rubbers will influence the rate and degree of vulcanization and, in consequence, the performance of the final composite. In this work, natural rubber (NR) and styrene butadiene rubber (SBR) were mixed in a 1:1 ratio. The compositions were obtained according to ASTM D 3182, by using four different preparative modes for the incorporation of the additives. After vulcanization, morphological, and dynamic mechanical thermal analysis, tensile strength, hardness, and tear resistance of each composition were investigated. The results show that the best properties were found when the NR/SBR mixture was prepared in such a way as to favor the vulcanization of the SBR phase while preserving the NR phase from excessive vulcanization. (C) 2004 Wiley Periodicals, Inc.