Process for preparing isocyanates by reacting the corresponding amines comprised in at least one feed stream A with phosgene comprised in at least one feed stream P in a reaction plant comprising at least one mixing zone and at least one reaction zone, wherein feed stream A and/or feed stream P optionally comprise one or more inert materials and,during periods of time in which the flow Sx of the amine used is below the flow S0 of the amines used during operation at the nominal capacity of the reactor plant, (i) the ratio of phosgene to amine is increased and/or (ii) the concentration of the inert material or materials in the amine-comprising feed stream A and/or the phosgene-comprising feed stream P is increased compared to operation at the nominal capacity of the reactor.