The present invention relates to a process for preparing isocyanates and an apparatus suitable for this purpose, and also its use. In the process, an amine and phosgene, both in the gas phase, react in the presence of an inert medium. A number, n, of amine streams are reacted with a number, n +1, of phosgene streams in a reactor. The number n is a positive integer of at least 1. All amine and phosgene streams are introduced into the reactor through annular gaps for mixing.