Polyolefin composition comprising, in percent by weight: A) 10-50% of a copolymer of propylene with one or more comonomer(s) selected from ethylene and CH2═CHR alpha-olefins where R is a 2-8 carbon alkyl, which copolymer contains from 1 to 8% of comonomer(s); B) 50-90% of a blend of copolymers (bI) and (bII), wherein copolymer (bI) contains from 18 to 40% of ethylene, and copolymer (bII) contains from 55 to 85% of ethylene, and the weight ratio (bI)/(bII) is from 0.2 to 2; in which composition the copolymer component (bI) is present in amounts of 15% or more with respect to the total weight of (A)+(B), the weight ratio C2II/C2I of the content C2II of ethylene in copolymer component (bII) to the content C2I of ethylene in copolymer component (bI) is of 1.8 or more, and the intrinsic viscosity [η] of the fraction XS soluble in xylene at room temperature (about 25° C.), referred to the total weight of (A)+(B), is of 2 dl/g or more.