A process for the continuous preparation of adiponitrile by hydrocyanation of 3-pentenenitrile, wherein a) 3-pentenenitrile or a mixture comprising 3-pentenenitrile is hydrocyanated by means of hydrocyanic acid in the presence of at least one nickel complex as catalyst, free ligand, and at least one Lewis acid in a reactor R1 to give a reaction output comprising adiponitrile, 2-methylglutaronitrile, the nickel complex, free ligand, Lewis acid, catalyst degradation products, unreacted 3-pentenenitrile and, as secondary components, cis- and trans-2-methyl-2-butenenitrile and also cis- and trans-2-pentenenitrile, b) in a work-up 1, a mixture comprising cis-2-methyl-2-butenenitrile and cis-2-pentenenitrile is separated off as overhead product, a mixture comprising adiponitrile, nickel complex, free ligand, at least one Lewis acid and catalyst degradation products is separated off as bottom product and a mixture comprising trans-2-methyl-2-butenenitrile, trans-2-pentenenitrile and 3-pentenenitrile is separated off at a side offtake from the reaction output from the reactor R1 in a first distillation apparatus, the side offtake product is fractionated in a second distillation apparatus in such a way that trans-2-methyl-2-butenenitrile is separated off as overhead product and discharged and 3-pentenenitrile and trans-2-pentenenitrile are obtained as bottom product and recirculated to the 3-pentenenitrile hydrocyanation in step a), c) the mixture comprising cis-2-methyl-2-butenenitrile and cis-2-pentenenitrile from step b) is continuously isomerized in the presence of aluminum oxide as catalyst in a reaction zone in a reactive distillation column R2 to give a product mixture comprising 3-pentenenitrile, where the isomerization is carried out at temperatures of from 150 to 220°C and pressures of from 1 to 15 bar in the liquid phase and the aluminum oxide has a BET surface area of from 50 to 450 m 2 /g and a pH of from 7 to 10.5, and cis-2-methyl-2-butenenitrile is separated off as overhead product and discharged, unreacted cis-2-pentenenitrile is separated off from a side offtake and recirculated to the reactor R2 in step c) and the 3-pentenenitrile-comprising bottom product is recirculated to the 3-pentenenitrile hydrocyanation in step a). a) 3-pentenenitrile or a mixture comprising 3-pentenenitrile is hydrocyanated by means of hydrocyanic acid in the presence of at least one nickel complex as catalyst, free ligand, and at least one Lewis acid in a reactor R1 to give a reaction output comprising adiponitrile, 2-methylglutaronitrile, the nickel complex, free ligand, Lewis acid, catalyst degradation products, unreacted 3-pentenenitrile and, as secondary components, cis- and trans-2-methyl-2-butenenitrile and also cis- and trans-2-pentenenitrile, b) in a work-up 1, a mixture comprising cis-2-methyl-2-butenenitrile and cis-2-pentenenitrile is separated off as overhead product, a mixture comprising adiponitrile, nickel complex, free ligand, at least one Lewis acid and catalyst degradation products is separated off as bottom product and a mixture comprising trans-2-methyl-2-butenenitrile, trans-2-pentenenitrile and 3-pentenenitrile is separated off at a side offtake from the reaction output from the reactor R1 in a first distillation apparatus, the side offtake product is fractionated in a second distillation apparatus in such a way that trans-2-methyl-2-butenenitrile is separated off as overhead product and discharged and 3-pentenenitrile and trans-2-pentenenitrile are obtained as bottom product and recirculated to the 3-pentenenitrile hydrocyanation in step a), c) the mixture comprising cis-2-methyl-2-butenenitrile and cis-2-pentenenitrile from step b) is continuously isomerized in the presence of aluminum oxide as catalyst in a reaction zone in a reactive distillation column R2 to give a product mixture comprising 3-pentenenitrile, where the isomerization is carried out at temperatures of from 150 to 220°C and pressures of from 1 to 15 bar in the liquid phase and the aluminum oxide has a BET surface area of from 50 to 450 m 2 /g and a pH of from 7 to 10.5, and cis-2-methyl-2-butenenitrile is separated off as overhead product and discharged, unreacted cis-2-pentenenitrile is separated off from a side offtake and recirculated to the reactor R2 in step c) and the 3-pentenenitrile-comprising bottom product is recirculated to the 3-pentenenitrile hydrocyanation in step a).