A foamable thermoplastic fluoropolymer composition comprising: - at least one melt-processible fluoropolymer [polymer (A)], said polymer (A) possessing a dynamic viscosity at a shear rate of 1 rad x sec -1 and at a temperature of T m2 + (30 ± 2°C) comprised between 10 and 10 6 Pa × sec, when measured with a controlled strain rheometer, employing an actuator to apply a deforming strain to the sample and a separate transducer to measure the resultant stress developed within the sample, and using the parallel plate fixture; and - at least one perfluoroalkoxysulphonic derivative selected from following compounds: (i) CF 3 -(CF 2 ) w -OCF 2 CF 2 -SO 3 - (Z p+ ) 1/p wherein w is an integer from 1 to 3, preferably w = 1, and Z is NH 4 or a alkaline or alkali-earth metal cation, and p being the valence of the cation Z; (ii) CF 3 -(CF 2 ) w' OCF 2 CF 2 -OCF 2 CF 2 -SO 3 -(Z p+ ) 1/p wherein w' is an integer from 0 to 2, preferably w' = 0, and Z is NH 4 or a alkaline or alkali-earth metal cation, and p being the valence of the cation Z; and (iii) CF 3 -(CF 2 ) q -(OCF 2 CFX F ) r -OCF 2 CF 2 -SO 3 - (Z p+ ) 1/p wherein q and r being integers from 1 to 3, preferably q = 1 and r = 1, X F is F or CF 3 and Z is NH 4 or a alkaline or alkali-earth metal cation, and p being the valence of the cation Z. - at least one melt-processible fluoropolymer [polymer (A)], said polymer (A) possessing a dynamic viscosity at a shear rate of 1 rad x sec -1 and at a temperature of T m2 + (30 ± 2°C) comprised between 10 and 10 6 Pa × sec, when measured with a controlled strain rheometer, employing an actuator to apply a deforming strain to the sample and a separate transducer to measure the resultant stress developed within the sample, and using the parallel plate fixture; and - at least one perfluoroalkoxysulphonic derivative selected from following compounds: (i) CF 3 -(CF 2 ) w -OCF 2 CF 2 -SO 3 - (Z p+ ) 1/p wherein w is an integer from 1 to 3, preferably w = 1, and Z is NH 4 or a alkaline or alkali-earth metal cation, and p being the valence of the cation Z; (ii) CF 3 -(CF 2 ) w' OCF 2 CF 2 -OCF 2 CF 2 -SO 3 -(Z p+ ) 1/p wherein w' is an integer from 0 to 2, preferably w' = 0, and Z is NH 4 or a alkaline or alkali-earth metal cation, and p being the valence of the cation Z; and (iii) CF 3 -(CF 2 ) q -(OCF 2 CFX F ) r -OCF 2 CF 2 -SO 3 - (Z p+ ) 1/p wherein q and r being integers from 1 to 3, preferably q = 1 and r = 1, X F is F or CF 3 and Z is NH 4 or a alkaline or alkali-earth metal cation, and p being the valence of the cation Z. (i) CF 3 -(CF 2 ) w -OCF 2 CF 2 -SO 3 - (Z p+ ) 1/p wherein w is an integer from 1 to 3, preferably w = 1, and Z is NH 4 or a alkaline or alkali-earth metal cation, and p being the valence of the cation Z; (ii) CF 3 -(CF 2 ) w' OCF 2 CF 2 -OCF 2 CF 2 -SO 3 -(Z p+ ) 1/p wherein w' is an integer from 0 to 2, preferably w' = 0, and Z is NH 4 or a alkaline or alkali-earth metal cation, and p being the valence of the cation Z; and (iii) CF 3 -(CF 2 ) q -(OCF 2 CFX F ) r -OCF 2 CF 2 -SO 3 - (Z p+ ) 1/p wherein q and r being integers from 1 to 3, preferably q = 1 and r = 1, X F is F or CF 3 and Z is NH 4 or a alkaline or alkali-earth metal cation, and p being the valence of the cation Z. <EOS>