A polyolefin composition, and pipe systems and sheets made therefrom, the composition comprising (percent by weight), 1) 10-60% of a copolymer of propylene and hexene-1 comprising from 0.2 to 10% of recurring units derived from hexene-1; 2) 10-85% of a propylene polymer selected from propylene homopolymer and a polymer of propylene with 0.1-10% of an α-olefin selected from ethylene, a C4-C10 a-olefin, hexene-1 excluded, and a mixture thereof, the propylene polymer being insoluble in xylene at ambient temperature in an amount over 85% and having a Polydispersity Index ranging from 4.5 to 12; and 3) 5-30% of a copolymer of ethylene with a C3-C10 α-olefin and optionally a diene, having an ethylene content ranging from 15 to 60% and an intrinsic viscosity value of at least 1 dl/g.