N-chlorocarbonyl isocyanide dichloride is prepared by chlorinating methyl isocyanate and/or methyl carbamic acid chloride in solution in the presence of ultraviolet light at a temperature in the range of from 30.degree. to 120.degree.C. The temperature pattern during the reaction is controlled in relation to the concentration of methyl isocyanate in the reaction mixture such that low temperatures are maintained for high concentrations while high temperatures are maintained with low concentrations. Thus, when a temperature of from 30.degree.-40.degree.C is maintained during the reaction, the concentration of methyl isocyanate in the reaction mixture is substantially more than 50 percent; when the temperature is increased to 65.degree. to 75.degree.C, the concentration of methyl isocyanate is in the range of 50 percent to 10 percent; and when the temperature is increased above 75.degree.C, the concentration of methyl isocyanate is substantially below 10 percent.