ClCONCO (N-chlorocarbonyl isocyanate) is prepared in 90% yield by partial hydrolysis of the addition product of phosgene and cyanogen chloride. Many derivatives of the iminocarboxylic acid parent compound can be obtained from this highly reactive species. Both the acid chloride and the isocyanate group are amenable to reactions exhibiting overall selctivity, and this opens the way, inter alia, to a simple preparative synthesis of isocyanates from alcohols, phenols, thiols, and thiophenols. Combination of the two functional groups and their peculiar symmetry, which becomes evident, e.g. in the HCl adduct, facilitates ready cyclization of N-chlorocarbonyl isocyanate under very mild conditons.