A series of particulate composites based on a new family of propylene/ethylene (P/E) copolymers produced by Dow Chemical Co. were studied with microscopy and differential scanning calorimetry (DSC). To understand the good processability exhibited by the composites, we studied the composite microstructure and the bulk rheological properties. Here we report the results of a study of the microstructure and thermal behavior. Electron micrographs of the composites showed a uniform dispersion of the particles in the copolymer matrices at all experimental concentrations with very little particle agglomeration. The images suggested low adhesion between the matrices and the particles. The copolymers were semicrystalline, and their morphologies changed with the ethylene content. An increase in the ethylene content led to a decrease in the crystallinity and changes in the shape and size of the crystallites. Tapping-mode atomic force microcopy images of a 9 wt % ethylene copolymer contained spherulites, lamellae, and a crosshatched structure characteristic of a-isotactic polypropylene. The crosshatched structure was not present at higher ethylene contents. A DSC study of the pure copolymers and their composites revealed that only very small modifications to the thermal behavior of the P/E copolymers were induced by calcium carbonate particles. (c) 2007 Wiley Periodicals, Inc.