Process for cleaning a phosgene-conducting apparatus, comprising the steps of: a) providing a device for gas removal from the apparatus to be cleaned and a device for gas supply to the apparatus to be cleaned and shutting off all conduits that may additionally be connected to the apparatus to be cleaned; b) passing an inert gas having a temperature in the range from 100°C to 500°C through the apparatus to be cleaned; c) stopping the inert gas stream from step b), followed by cooling of the apparatus to be cleaned to a temperature fixed beforehand, measured in the device for gas removal, in the range from 0°C to 70°C by passing through an inert gas at a temperature equal to or below the temperature fixed beforehand, where the inert gas stream from step c) may additionally also be maintained even after cooling of the apparatus to be cleaned; d) once the apparatus to be cleaned has been cooled in step c), passing through gaseous ammonia from an ammonia gas source to obtain an ammonia-containing gas mixture, where the flow rate of the gaseous ammonia is chosen such that the temperature in the gas mixture removed from the apparatus to be cleaned rises continuously and drops again after reaching a maximum, where the temperature maximum is in the range from 30°C to 120°C; e) no earlier than after attainment of the temperature maximum in step d), removing the ammonia gas source from the apparatus to be cleaned, followed by passing of an inert gas through the apparatus to be cleaned; f) optionally, after stopping the inert gas stream, providing a liquid inlet into the apparatus to be cleaned and a liquid outlet out of the apparatus to be cleaned and passing an aqueous stream through. a) providing a device for gas removal from the apparatus to be cleaned and a device for gas supply to the apparatus to be cleaned and shutting off all conduits that may additionally be connected to the apparatus to be cleaned; b) passing an inert gas having a temperature in the range from 100°C to 500°C through the apparatus to be cleaned; c) stopping the inert gas stream from step b), followed by cooling of the apparatus to be cleaned to a temperature fixed beforehand, measured in the device for gas removal, in the range from 0°C to 70°C by passing through an inert gas at a temperature equal to or below the temperature fixed beforehand, where the inert gas stream from step c) may additionally also be maintained even after cooling of the apparatus to be cleaned; d) once the apparatus to be cleaned has been cooled in step c), passing through gaseous ammonia from an ammonia gas source to obtain an ammonia-containing gas mixture, where the flow rate of the gaseous ammonia is chosen such that the temperature in the gas mixture removed from the apparatus to be cleaned rises continuously and drops again after reaching a maximum, where the temperature maximum is in the range from 30°C to 120°C; e) no earlier than after attainment of the temperature maximum in step d), removing the ammonia gas source from the apparatus to be cleaned, followed by passing of an inert gas through the apparatus to be cleaned; f) optionally, after stopping the inert gas stream, providing a liquid inlet into the apparatus to be cleaned and a liquid outlet out of the apparatus to be cleaned and passing an aqueous stream through.