PURPOSE: To prepare isocyanate useful for intermediates of plastics, lacquers and plant protective agents in a simply, a high purity and a high yield process by decomposing N,N,N'-trisubstituted ureas.

CONSTITUTION: The process comprises decomposing N,N,N'-trisubstituted ureas of the formula (wherein R1, R2 and R3 are each 1-24C alkyl radical, R2 and R3 are together bonded to form a heterocyclic un-aromatic radical) at a temperature of 150 to 300°C and under a reduced pressure of 0.01 to 100 mbar to produce a high volatile isocyanate of the formula R1NCO and a low volatile secondary amine of the formula R2R3NH, and then drawing off the isocyanate at the top by means of a stream of carrier gas. N,N-dioctyl-N'-ethyl urea is exampled for the starting compound of the formula.