A mixture comprising a bitumen (A), and a polymer composition (B) comprising the following components, all percent amounts being by weight: I) from 1 to 30% of a butene-1 polymer having a Melt Flow Rate (MFR), measured according to ASTM D 1238, at 190° C./2.16 kg, from 0.1 to 1000 g/10 min.; and II) from 70 to 99% of a heterophasic polyolefm composition comprising the following polymer fractions: a) 4-45% of a propylene homopolymer or copolymer; b) 0-20% of a of a copolymer fraction containing ethylene, insoluble in xylene at room temperature; c) 40-95% of a copolymer of ethylene with propylene and/or a CH2═CHR α-olefin, where R is a C2-C8 alkyl radical, containing ethylene in a quantity lower than 40% and being soluble in xylene at room temperature.