Calcium carbonate-filled polypropylene composites were processed on two different high-intensity mixing devices. The resulting relationships between morphology, processing conditions and mechanical properties of these composites were studied using an electron microscope with an image-analysis system and impact and tensile testing. Only under more extreme conditions did the recently developed turbine mixer give as good a dispersion as the classical Banbury-style mixer. Relatively small additions of the filler resulted in small increases in absorbed impact energy. Semi-quantitative information about the randomness achieved in the mixing process was obtained using a dilation/counting procedure and computer-derived spatial images. Object-specific parameters such as the percentage of objects greater than 1 mum in size give good correlation with the mechanical properties of the mixture.