The microhardness, H, of carbon black-polycarbonate and carbon black-low-density polyethylene composites was investigated. Two types of microadditives with different average particle sizes were employed. It has been shown that the morphology of the polymeric matrix conspicuously influences the hardness dependence of the composites with volume concentration of filler, phi. The microhardness of the carbon black-polycarbonate composites shows a steplike behavior with respect to carbon black content, while the H value of the carbon black-low-density polyethylene composite linearly increases with increasing phi. The influence of filler structure on the microhardness of the carbon black-polymer composites is also discussed. Results favor the concept that a smaller carbon black particle size (smaller aggregate diameters and interaggregate distances) enhances the microhardness of the composites.