A process for preparing a diamine from a corresponding aminoalkyl nitrile, which is prepared by reaction of a corresponding monoamine with a corresponding alkenyl nitrile in a continuous mode of operation, comprising the steps: a) introduction of the corresponding monoamine into a continuously conveyed reaction stream; b) introduction of the corresponding alkenyl nitrile into the reaction stream, with this already comprising the aminoalkyl nitrile on addition; c) reaction of the reaction stream in a first reaction region; d) at least partial transfer of the reaction stream into at least one second reaction region for further reaction; e) discharge of the reaction stream from the second reaction region after the reaction; f) introduction of the reaction stream discharged in step (a) into a reduction region; and g) reduction of the aminoalkyl nitrile present in the reaction stream to the corresponding diamine.