A process for the production of a carbon fiber precursor comprising the following steps: i) preparation of an acid comonomer solution selected from itaconic acid or acrylic acid in water containing at least a stoichiometric quantity of ammonia with respect to the acid groups present, wherein the concentration of acid comonomer in water varies within the range of 3% to 50% by weight, the total quantity of water fed to the reactor ranging from 1 to 5% by weight with respect to the total weight of the mass charged into the reactor in step ii); ii) feeding into the reactor, acrylonitrile or a mixture of acrylonitrile and a neutral vinyl comonomer, the two comonomers acrylonitrile/neutral vinyl comonomer being present in a weight ratio ranging from 95:5 to 99.5:0.5, the aqueous solution of ammonium salt of the acid comonomer prepared in step i), DMSO, 2-2'-azobisisobutyronitrile AIBN and dodecylmercaptan or octylmercaptan; iii) keeping the mixture thus obtained under stirring at a temperature ranging from 50 to 80°C, for a time ranging from 10 to 20 hours, preferably from 12 to 15 hours; iv) discharging the contents of the reactor into a tank kept at a temperature ranging from 35 to 40°C to slow down or interrupt the reaction; v) feeding the mixture thus obtained to a thin-film evaporation system (TFE) operating at a temperature ranging from 40 to 80°C, and at a pressure ranging from 5 to 30 mbar abs; vi) collecting a mixture of acrylonitrile, water and DMSO from the head of the TFE, said mixture being re-used in the preparation of a new reaction batch; vii) collecting a polymer solution in DMSO from the bottom of the TFE, said solution being diluted with fresh DMSO to reach a concentration ranging from 15 to 25% by weight, preferably from 18 to 22% by weight, with respect to the total weight of the solution ; viii) feeding the homogeneous spinning solution obtained at the end of step vii) to the spinning step or to a storage tank. i) preparation of an acid comonomer solution selected from itaconic acid or acrylic acid in water containing at least a stoichiometric quantity of ammonia with respect to the acid groups present, wherein the concentration of acid comonomer in water varies within the range of 3% to 50% by weight, the total quantity of water fed to the reactor ranging from 1 to 5% by weight with respect to the total weight of the mass charged into the reactor in step ii); ii) feeding into the reactor, acrylonitrile or a mixture of acrylonitrile and a neutral vinyl comonomer, the two comonomers acrylonitrile/neutral vinyl comonomer being present in a weight ratio ranging from 95:5 to 99.5:0.5, the aqueous solution of ammonium salt of the acid comonomer prepared in step i), DMSO, 2-2'-azobisisobutyronitrile AIBN and dodecylmercaptan or octylmercaptan; iii) keeping the mixture thus obtained under stirring at a temperature ranging from 50 to 80°C, for a time ranging from 10 to 20 hours, preferably from 12 to 15 hours; iv) discharging the contents of the reactor into a tank kept at a temperature ranging from 35 to 40°C to slow down or interrupt the reaction; v) feeding the mixture thus obtained to a thin-film evaporation system (TFE) operating at a temperature ranging from 40 to 80°C, and at a pressure ranging from 5 to 30 mbar abs; vi) collecting a mixture of acrylonitrile, water and DMSO from the head of the TFE, said mixture being re-used in the preparation of a new reaction batch; vii) collecting a polymer solution in DMSO from the bottom of the TFE, said solution being diluted with fresh DMSO to reach a concentration ranging from 15 to 25% by weight, preferably from 18 to 22% by weight, with respect to the total weight of the solution ; viii) feeding the homogeneous spinning solution obtained at the end of step vii) to the spinning step or to a storage tank. <EOS>