Solid, homogeneous, particulate, highly-filled polyolefin composites which comprise(a) about 10-75% by weight of polyolefin having an inherent viscosity of at least about 2, and(b) about 25-90% by weight of finely-divided, inorganic filler compound having catalytically-active transition metal compound interacted at its surface,said polyolefin being polymerized onto the surface of said filler, and said composite having a 10-second micronization homogeneity of at least about 50% and a micronization homogeneity index of at least about 20. Methods of preparing these composites and methods of forming objects having outstanding properties from these composites are described.