Polybutadiene (BR) and styrene-butadiene (SBR) rubbers containing the same loading of precipitated silica nanofiller were prepared. The silica surfaces were pretreated with bis(3-triethoxysilylpropyl) tetrasulfide to chemically bond the silica to the rubber. The rubber compounds were mixed together for different times and at different temperatures to produce SBR/BR blends. The mass fraction and composition values of the interphases in the blends were subsequently determined with modulated-temperature differential scanning calorimetry. These properties changed substantially as a function of mixing temperature and mixing time. The hardness, tensile strength, elongation at break, stored energy density at break, tear strength, modulus, abrasion resistance, heat buildup, and loss tangent of the cured blends were measured over a wide range of test conditions. Elongation at break, stored energy density at break, tearing energy, and abrasion resistance benefited from increases in the mass fraction of the interphase. The remaining properties were influenced mainly by the filler loading and mixing time of the two rubber compounds. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113:1868-1878,2009