The invention relates to a process for preparing an isocyanate, which involves contacting fluid streams of amine, phosgene and inert medium in at least one mixing device, such that an inert medium stream is metered at least between one amine stream and one phosgene stream within the mixing device, and then reacting an amine with phosgene in a reaction chamber to form an isocyanate. In this process the inert medium stream is metered such that a point of first contact of the amine and the phosgene occurs at a distance from a surface of the mixing device, and a turbulent flow is present in the reaction chamber.