A polyolefin composition particularly suited for producing geomembranes, comprising: A) from 55 to 75 wt% of a polyethylene having a density of 0.920-0.940 g/cm3 and a fraction soluble in xylene (XSA) of less than 10%; B) from 25 to 45 wt. % of a copolymer of ethylene with one or more comonomers selected from α-olefins having formula HC2=CHR1, where R1 is an alkyl radical, linear or branched, having from 1 to 8 carbon atoms, comprising from 50 to 70 wt.% of ethylene and having a fraction soluble in xylene (XSB) of at least 50 wt.%; said composition having a flexural modulus of less than 260 MPa as measured according to ISO 178.