The present invention is a process for aliphatic or cycloaliphatic isocyanate. The process comprises reacting an aliphatic or cycloaliphatic primary amine, with phosgene in the presence of an inert solvent wherein the initial reaction temperature is between 100 and 130° C. and the temperature is subsequently ramped to 150 to 180° C. during the course of the reaction, the solvent to amine weight ratio is 95:5 to 80:20, the total reaction pressure is maintained between 50 and 350 psig and the amine is rapidly dispersed in the phosgene through injection in a region of high efficiency mixing.