Tensile properties and Izod impact strength of mica-filled composites of poly(butylene terephthlate) (PBT)/polyacrylonitrile-butyl acrylate-styrene (ABAS) were studied at mica concentration range 0 to 0.14 volume fraction, (phi(f)). Tensile properties such as tensile modulus, strength, and breaking strain were normalized by dividing the data with the crystallinity (%) of the major component PBT in the composites and the matrix blends. The normalized relative tensile properties were compared with simple models to evaluate the interphase interactions between the matrix (i.e. PBT/ABAS blend) and the dispersed phase mica. Mica reinforced the blend increasing the tensile modulus and strength with mica concentration while the strain-at-break was increased marginally up to phi(f) = 0.04 and decreased beyond this phi(f). The impact strength, however, decreased with increase in phi(f) due to enhanced matrix stiffening and lack of plastic deformation of the matrix. Scanning electron microscopic studies revealed good dispersion of mica in the composites. The effect of surface treatment with a zirconate coupling agent, NZ-97, on the above properties has also been examined. (C) 2010 Wiley Periodicals, Inc. J April Polym Sci 117: 672-681, 2010