Abstract The preparation of [14 CO] methyl isocyanate from [1-14 C] acetyl chloride and activated sodium azide through the Curtius rearrangement was optimized (50% yield/[1-14 C] sodium acetate-50 mCi/mmol) and the reagent immediately used for the synthesis of [14 CO] eserine,[14 CO] carbaryl and [14 CO] IPMU.