Process for preparing an isocyanate by reacting a primary amine with phosgene, comprising the steps of: I) providing a solution of the primary amine in a solvent; II) providing a solution of phosgene in a solvent; III) mixing the solution of the primary amine provided in step I) and the solution of phosgene provided in step II) in a mixing unit to give a reaction mixture of a temperature in the range from 110°C to 145°C with observance of a stoichiometric excess of phosgene based on the amino groups of the primary amine in the range from 40% to 200% of theory; IV) running the reaction mixture obtained in step III) through a reaction zone and through a separation zone downstream in flow direction of this reaction zone to form a gas phase under a pressure in the range from 8.0 bar( abs.) to 50.0 bar( abs.) from the liquid reaction mixture in the separation zone, where the reaction zone and the separation zone are not heated and not cooled, where the gas phase formed in the separation zone and the remaining liquid phase are removed from the separation zone separately from one another; V) expanding the liquid phase withdrawn from the separation zone from step IV) with partial conversion of this liquid phase to the gas phase; VI) running the liquid phase that remains after the expansion in step V) through an indirectly heated reaction zone, thereby forming a hydrogen chloride- and phosgene-containing gas phase which is removed, and whereby an isocyanate- and solvent-containing liquid phase remains which is withdrawn from the indirectly heated reaction zone; VII) working up the isocyanate- and solvent-containing liquid phase obtained in step VI) to recover the solvent and obtain the isocyanate; characterized in that in step V) the liquid phase withdrawn from the separation zone from step IV) is expanded by (i) first expanding this liquid phase in a first gas-liquid separation vessel, forming a first liquid phase and a first gas phase, (ii) then subsequently further expanding the first liquid phase in a second gas-liquid separation vessel, forming a second liquid phase and a second gas phase, (iii) then subsequently optionally further expanding the second liquid phase in a third gas-liquid separation vessel, forming a third liquid phase and a third gas phase, where the liquid phase run through the indirectly heated reaction zone in step VI) is the second or the third liquid phase. I) providing a solution of the primary amine in a solvent; II) providing a solution of phosgene in a solvent; III) mixing the solution of the primary amine provided in step I) and the solution of phosgene provided in step II) in a mixing unit to give a reaction mixture of a temperature in the range from 110°C to 145°C with observance of a stoichiometric excess of phosgene based on the amino groups of the primary amine in the range from 40% to 200% of theory; IV) running the reaction mixture obtained in step III) through a reaction zone and through a separation zone downstream in flow direction of this reaction zone to form a gas phase under a pressure in the range from 8.0 bar( abs.) to 50.0 bar( abs.) from the liquid reaction mixture in the separation zone, where the reaction zone and the separation zone are not heated and not cooled, where the gas phase formed in the separation zone and the remaining liquid phase are removed from the separation zone separately from one another; V) expanding the liquid phase withdrawn from the separation zone from step IV) with partial conversion of this liquid phase to the gas phase; VI) running the liquid phase that remains after the expansion in step V) through an indirectly heated reaction zone, thereby forming a hydrogen chloride- and phosgene-containing gas phase which is removed, and whereby an isocyanate- and solvent-containing liquid phase remains which is withdrawn from the indirectly heated reaction zone; VII) working up the isocyanate- and solvent-containing liquid phase obtained in step VI) to recover the solvent and obtain the isocyanate; (i) first expanding this liquid phase in a first gas-liquid separation vessel, forming a first liquid phase and a first gas phase, (ii) then subsequently further expanding the first liquid phase in a second gas-liquid separation vessel, forming a second liquid phase and a second gas phase, (iii) then subsequently optionally further expanding the second liquid phase in a third gas-liquid separation vessel, forming a third liquid phase and a third gas phase,