Method of operating a gas phase phosgenation plant for reacting an amine with phosgene in a stoichiometric excess in relation to the primary amino groups of the amine to give the corresponding isocyanate, said gas phase phosgenation plant comprising at least (i) an apparatus 1000 for providing a gaseous phosgene stream, optionally comprising, as well as phosgene, an inert substance, (ii) an apparatus 2000 for providing a gaseous amine stream, optionally comprising, as well as amine, an inert substance, (iii) a mixing zone for mixing the streams and, the mixing zone being connected by each of devices ( 1100, 2100) to the apparatus 1000 and the apparatus 2000, (iv) a reaction zone arranged downstream of the mixing zone for further conversion of the previously mixed streams and, (v) a reaction stopping zone arranged downstream of the reaction zone to end the reaction, and optionally (vi) a workup section comprising devices for recovery and recyling of unconverted phosgene ( 1") and devices for obtaining the isocyanate prepared in pure form, characterized in that the gas phase phosgenation plant is shut down by running the following steps: reducing the amine mass flow rate M' to zero, while maintaining a gaseous inert gas stream through the device 2100, the mixing zone, the reaction zone and the reaction stopping zone; only after a period corresponding to at least 10 times the residence time, preferably at least 30 times the residence time, more preferably at least 60 times the residence time and most preferably at least 90 times the residence time of phosgene from exit from the apparatus 1000 to exit from the reaction stopping zone in the regular operation of the gas phase phosgenation plant has elapsed, calculated from the moment from which M' is zero: reducing the mass flow rate of phosgene M' leaving the apparatus 1000 to zero, while maintaining a gaseous inert gas stream through the device 1100, the mixing zone, the reaction zone and the reaction stopping zone; maintaining at least the inert gas stream from, preferably the inert gas streams from and, at least for a period corresponding to three times, preferably 100 times, more preferably 200 times and most preferably 350 times the residence time of the inert gas stream from entry into the mixing zone to exit from the reaction stopping zone, calculated from the moment from which the mass flow rate of phosgene M' leaving the apparatus 1000 is zero. (i) an apparatus 1000 for providing a gaseous phosgene stream, optionally comprising, as well as phosgene, an inert substance, (ii) an apparatus 2000 for providing a gaseous amine stream, optionally comprising, as well as amine, an inert substance, (iii) a mixing zone for mixing the streams and, the mixing zone being connected by each of devices ( 1100, 2100) to the apparatus 1000 and the apparatus 2000, (iv) a reaction zone arranged downstream of the mixing zone for further conversion of the previously mixed streams and, (v) a reaction stopping zone arranged downstream of the reaction zone to end the reaction, and optionally (vi) a workup section comprising devices for recovery and recyling of unconverted phosgene ( 1") and devices for obtaining the isocyanate prepared in pure form, reducing the amine mass flow rate M' to zero, while maintaining a gaseous inert gas stream through the device 2100, the mixing zone, the reaction zone and the reaction stopping zone; only after a period corresponding to at least 10 times the residence time, preferably at least 30 times the residence time, more preferably at least 60 times the residence time and most preferably at least 90 times the residence time of phosgene from exit from the apparatus 1000 to exit from the reaction stopping zone in the regular operation of the gas phase phosgenation plant has elapsed, calculated from the moment from which M' is zero: reducing the mass flow rate of phosgene M' leaving the apparatus 1000 to zero, while maintaining a gaseous inert gas stream through the device 1100, the mixing zone, the reaction zone and the reaction stopping zone; maintaining at least the inert gas stream from, preferably the inert gas streams from and, at least for a period corresponding to three times, preferably 100 times, more preferably 200 times and most preferably 350 times the residence time of the inert gas stream from entry into the mixing zone to exit from the reaction stopping zone, calculated from the moment from which the mass flow rate of phosgene M' leaving the apparatus 1000 is zero.