Polyolefin compositions comprising (percent by weight): 1) 55-90% of a crystalline propylene homopolymer or copolymer containing up to 15% of ethylene and/or C4-C10 α-olefin(s) and having values of MFR equal to or higher than 25 g/10 min; 2) 10-45% of a copolymer of ethylene with one or more C4-C10 α-olefin(s) containing from 10 to 40% of said C4-C10 α-olefin(s); said compositions having values of MFR equal to or higher than 20 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, a total fraction soluble in xylene at room temperature of less than 18 wt % and an intrinsic viscosity value of the fraction soluble in xylene at room temperature of 1.7 dl/g or less.