Ethylene-propylene copolymers have been prepared by using Ziegler-Natta catalysts based on TiCl4, MgCl2, PCl3 and (n-Bu)(3)PO4. The catalysts TiCl4/MgCl2/PCl3 and TiCL4/MgCl2/(n-Bu)(3)PO4 were prepared by reacting TiCl4 with pretreated MgCl2. The support was prepared by ball milling of MgCl2 with varied amounts of PCl3 or (n-Bu)(3)PO4. The addition of PCl3 has remarkably increased the MgCl2 surface area in comparison with (n-Bu)(3)PO4. The effects of PCl3 and (n-Bu)(3)PO4 on ethylene homopolymerization, ethylene-propylene copolymerization and on copolymer properties were evaluated. The catalyst system containing PCl3 permitted to synthesize propylene-ethylene copolymers with up to 75 % (w/w) of propylene and provided control of copolymer crystallinity. The reduction of the copolymer molecular weight distribution suggested that PCl3 acted as an internal donor, poisoning some active catalytic sites.