Polyethylene-based composites have been prepared by ethylene polymerization with a Al/Ti/Mg catalyst previously deposited on the surface of kaolin (Satintone W/W and Satintone 5) or barite.(1) Molecular weight (M(w)) of the polyethylene matrix has been estimated from melt viscosity under steady shear flow and compared to the value measured by size exclusion chromatography. Tensile stress and strain, Young modulus, and impact energy have been measured in relation to the matrix molecular weight and the filler content. The effect of the main polymerization parameters, such as the Al/Ti/Mg composition, the addition of a transfer agent, e.g., an alpha-olefin or hydrogen, have also been investigated. These tensile and impact properties have been compared to composites of the same composition but prepared by melt blending of the filler and the preformed polyolefin. As a rule, the polymerization-filled composites have an improved mechanical strength compared to the analogues prepared by melt blending.