The invention relates to a process for working up a stream (3) comprising from 5 to 90% by weight of isocyanate by hydrolysis of the isocyanate by means of water in a reactor (1), in which the isocyanate-comprising stream (3) is introduced to the reactor (1) via a first inlet and the water is introduced via a second inlet, the isocyanate is reacted with the water in the reactor to form the corresponding: diamine, a diamine-comprising product mixture is taken off from the reactor (1) via a product outlet, wherein part of the product mixture is recirculated to the reactor and the isocyanate-comprising stream (3) is introduced into the recirculated product stream (7), with the isocyanate-comprising stream (3) being introduced continuously without interruption and the molar ratio of H-acid compounds of the hydrolyzate to isocyanate groups of the stream introduced being kept in the range from 10 to 200.