Polyolefin compositions comprising (percent by weight): 1) 55-80% of a crystalline propylene homopolymer or copolymer containing up to 15% of ethylene and/or C4-C10 α-olefin(s) and having an MFR value of at least 15 g/10 min, and 2) 2) 20-45 wt % of a copolymer of ethylene with one or more C4-C10 α-olefin(s) containing from 10 to 4 wt % of said C4-C10 α-olefin(s). The said compositions have values of MFR equal to or higher than 15 g/10 min, a total content of ethylene of 20% or more, a total content of C4-C10 α-olefin(s) of 4.5% or more, a ratio of the total content of ethylene to the total content of C4-C10 α-olefin(s) of 2.3 or more, and an intrinsic viscosity value of the fraction soluble in xylene at room temperature of 1.7 dl/g or less.