Continuous process for preparing a diamine, comprising the steps of: (a) reaction between the corresponding alkenenitrile and the corresponding monoamine to form the corresponding aminonitrile, the monoamine being introduced in molar excess relative to the alkenenitrile, with recycling into the reaction of the unreacted monoamine; the recycling of the monoamine uses a separation system which makes it possible to recover: - on the one hand, the aminonitrile, a first fraction of which is sent to a reactor for step (b) and a second fraction of which is sent to an absorption system to enable dissolution of the excess monoamine; and - on the other hand, the excess monoamine in gaseous form, which is sent to said absorption system which enables its dissolution in said second fraction of aminonitrile. (b) reduction of the aminonitrile obtained in step (a) with hydrogen in the presence of at least one alkali metal hydroxide, water and a hydrogenation catalyst; (c) separation of the alkali metal hydroxide by evaporation of the diamine obtained in step (b) and of the water; (d) distillation of the diamine; - the alkenenitrile is chosen from linear or branched C 2 to C 4 alkenes in which a hydrogen atom is replaced with a conjugated cyano group and - the monoamine is a secondary amine of general formula R 1 R 2 NH, in which R 1 and R 2, which may be identical or different, represent a C 1 to C 4 alkyl. (a) reaction between the corresponding alkenenitrile and the corresponding monoamine to form the corresponding aminonitrile, the monoamine being introduced in molar excess relative to the alkenenitrile, with recycling into the reaction of the unreacted monoamine; the recycling of the monoamine uses a separation system which makes it possible to recover: - on the one hand, the aminonitrile, a first fraction of which is sent to a reactor for step (b) and a second fraction of which is sent to an absorption system to enable dissolution of the excess monoamine; and - on the other hand, the excess monoamine in gaseous form, which is sent to said absorption system which enables its dissolution in said second fraction of aminonitrile. - on the one hand, the aminonitrile, a first fraction of which is sent to a reactor for step (b) and a second fraction of which is sent to an absorption system to enable dissolution of the excess monoamine; and - on the other hand, the excess monoamine in gaseous form, which is sent to said absorption system which enables its dissolution in said second fraction of aminonitrile. (b) reduction of the aminonitrile obtained in step (a) with hydrogen in the presence of at least one alkali metal hydroxide, water and a hydrogenation catalyst; (c) separation of the alkali metal hydroxide by evaporation of the diamine obtained in step (b) and of the water; (d) distillation of the diamine; - the alkenenitrile is chosen from linear or branched C 2 to C 4 alkenes in which a hydrogen atom is replaced with a conjugated cyano group and - the monoamine is a secondary amine of general formula R 1 R 2 NH, in which R 1 and R 2, which may be identical or different, represent a C 1 to C 4 alkyl. - the alkenenitrile is chosen from linear or branched C 2 to C 4 alkenes in which a hydrogen atom is replaced with a conjugated cyano group and - the monoamine is a secondary amine of general formula R 1 R 2 NH, in which R 1 and R 2, which may be identical or different, represent a C 1 to C 4 alkyl.