A new family of propylene-ethylene copolymers developed by The Dow Chemical Company has narrow molecular weight distributions for industrial polymers and broad chemical composition distributions. This molecular architecture makes possible high filler loadings while maintaining good processability. To provide a fundamental understanding of the lower than expected viscosity, a study of the shear rheological behavior of two series of composites was performed. The composites consist of stearate-coated calcium carbonate particles suspended in a propylene-ethylene copolymer. The matrix in one series was a new copolymer, and in the other series, it was a traditional metallocene copolymer. For both systems, the viscosity increases dramatically with increasing filler loading; however, the viscosity is lower in the case of the composites of the new copolymer. The stearate coating on the calcium carbonate particles decreases the adhesion of the polymer to the filler surface, allowing particle-matrix interfacial slip. A high temperature atomic force microscopy study has indicated the existence of ethylene-enriched zones in the matrix immediately surrounding the particles in the new copolymer. We hypothesize that these high ethylene content chains around the particle surface enhance particle-matrix interfacial slip resulting in the lower composite viscosity.