Abstract The technique of in situ crystallization was utilized to fabricate a model composite system in which the filler morphology is variable under constant interface conditions. A butadiene-acrylonitrile copolymer was chosen as the matrix from which acetanilide was crystallized in two distinctly different crystal morphologies for filler volume loadings up to 0.35. At the same volume fraction filler, the shape of the relative modulus-filler loading curve is sensitive to the filler crystallization temperature and, thus, to the filler size and shape. For a given crystallization temperature (25°C), the data in the low-volume loading region (>0.2) can be represented reasonably well with the Mooney equation which indirectly yields a filler aspect ratio in agreement with scanning electron micrographs. When the observed morphological parameters are utilized for the entire filler loading range, the 25°C data are best predicted by the Halpin-Tsai equation in conjunction with lamination theory. In general, filler...