Polybutadiene and styrene-butadiene rubber compounds containing a high loading of a precipitated silica nanofiller were prepared. The silica surfaces were pretreated with bis(3-triethoxysilylpropyl) tetrasulfide to prevent the silica from interfering with the reaction mechanism of sulfur cure in the rubber. The rubber compounds here mixed together for different times and at different temperatures to produce styrene-butadiene rubber/polybutadiene rubber blends. The mass fraction and composition of the interphase in the blends were subsequently determined with modulated-temperature differential scanning calorimetry. At 60-65 degrees C, the mass fraction of the interphase in the blend increased after the rubbers were mixed together for 10 min, and then it decreased significantly when the mixing time was increased to 20 min. When the two rubbers were mixed together for 7 min at 60-105 degrees C, the mass fraction of the interphase in the blend increased slightly because of a higher mixing temperature. The composition of the interphase in the blend also changed with the mixing time and mixing temperature. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1641-1652, 2009