A process for the recovery and recycling of unreacted monomer in a single-step process for the production of a carbon-fiber precursor comprising the following steps: (a) discharging a reaction mixture comprising acrylonitrile polymers or copolymers mainly consisting of acrylonitrile and one or more co-monomers selected from vinyl molecules bearing one or more acid groups and optionally neutral vinyl molecules, water, polymerization solvent, unreacted monomers and possible reaction by-products, from a polymerization reactor into a tank kept at a temperature ranging from 35°C to 60°C in order to slow down or stop the polymerization reaction; (b) feeding the mixture deriving from step (a) to a thin-film evaporation system operating at a temperature ranging from 40°C to 80°C and at a pressure ranging from 10 to 30 mbar abs: (c) collecting a mixture of acrylonitrile, water and polymerization solvent from the head of the TFE; (d) feeding the mixture of acrylonitrile, water and polymerization solvent, collected from the head of the TFE in step c) at a temperature ranging from 60°C to 85°C, preferably 70-80°C, and at a pressure ranging from 10 to 30 mbar, preferably 15-25 mbar, to a first condenser cooled to a temperature of 30-45°C, preferably 36-42°C, obtaining a first liquid phase and a first vapour phase; (e) feeding the first vapour phase, exiting from said first condenser, to a second condenser cooled to a temperature of 12-25°C, preferably 12-20°C, obtaining a second liquid phase and a second vapour phase; (f) feeding the second vapour phase, exiting from said second condenser, to a third condenser cooled to a temperature ranging from -10 to 10°C, preferably from -5 to 0°C, obtaining a third liquid phase and a third vapour phase (g) combining the three liquid phases obtained in the previous steps d)-f), obtaining a fourth liquid phase comprising the polymerization solvent in a quantity ranging from 70 to 75% by weight, acrylonitrile in a quantity ranging from 24 to 29% by weight and water in a quantity equal to about 1% by weight, said percentages being indicated with respect to the total weight of the fourth liquid phase, said fourth liquid phase being then fed to a distillation column operating under vacuum at a pressure ranging from 300 to 500 mbar and kept at a temperature of 160°C or less; (h) collecting a fourth vapour phase from the head of the distillation column, which is fed to a purification column operating at reduced pressure ranging from 400 to 600 mbar and kept at a temperature of less than 70°C and recovering pure acrylonitrile from the bottom of the distillation column, which is recycled to the polymerization reactor. (a) discharging a reaction mixture comprising acrylonitrile polymers or copolymers mainly consisting of acrylonitrile and one or more co-monomers selected from vinyl molecules bearing one or more acid groups and optionally neutral vinyl molecules, water, polymerization solvent, unreacted monomers and possible reaction by-products, from a polymerization reactor into a tank kept at a temperature ranging from 35°C to 60°C in order to slow down or stop the polymerization reaction; (b) feeding the mixture deriving from step (a) to a thin-film evaporation system operating at a temperature ranging from 40°C to 80°C and at a pressure ranging from 10 to 30 mbar abs: (c) collecting a mixture of acrylonitrile, water and polymerization solvent from the head of the TFE; (d) feeding the mixture of acrylonitrile, water and polymerization solvent, collected from the head of the TFE in step c) at a temperature ranging from 60°C to 85°C, preferably 70-80°C, and at a pressure ranging from 10 to 30 mbar, preferably 15-25 mbar, to a first condenser cooled to a temperature of 30-45°C, preferably 36-42°C, obtaining a first liquid phase and a first vapour phase; (e) feeding the first vapour phase, exiting from said first condenser, to a second condenser cooled to a temperature of 12-25°C, preferably 12-20°C, obtaining a second liquid phase and a second vapour phase; (f) feeding the second vapour phase, exiting from said second condenser, to a third condenser cooled to a temperature ranging from -10 to 10°C, preferably from -5 to 0°C, obtaining a third liquid phase and a third vapour phase (g) combining the three liquid phases obtained in the previous steps d)-f), obtaining a fourth liquid phase comprising the polymerization solvent in a quantity ranging from 70 to 75% by weight, acrylonitrile in a quantity ranging from 24 to 29% by weight and water in a quantity equal to about 1% by weight, said percentages being indicated with respect to the total weight of the fourth liquid phase, said fourth liquid phase being then fed to a distillation column operating under vacuum at a pressure ranging from 300 to 500 mbar and kept at a temperature of 160°C or less; (h) collecting a fourth vapour phase from the head of the distillation column, which is fed to a purification column operating at reduced pressure ranging from 400 to 600 mbar and kept at a temperature of less than 70°C and recovering pure acrylonitrile from the bottom of the distillation column, which is recycled to the polymerization reactor.