The invention pertains to foamable thermoplastic fluoropolymer composition comprising: at least one fluoropolymer [polymer (A)]; and at least one perfluoroalkoxysulphonic derivative of either of formulae: (Xn+)1/n−O3S—CF2CF2O—Rdf—OCF2CF2—SO3−(Xn+)1/n and Rmf—OCF2CF2—SO3−(Xn+)1/n wherein Rdf is a divalent C1-C12 perfluorocarbon group, possibly comprising one or more ethereal oxygen atom; Rmf is a monovalent C1-C12 perfluorocarbon group, possibly comprising one or more ethereal oxygen atom, optionally comprising heteroatoms, generally selected from S, N, P; X=H, a metal cation, or an ammonium group; n is the valence of the cation X, preferably 1 or 2.