AbstractWhile [14C‐carbonyl]‐methyl isocyanate has been used widely for the preparation of radioactive insecticidal methylcarbamates, yields are often low, and in some cases reactions completely unsuccessful, because of the sensitivity of the isocyanate to moisture. This paper described a process whereby the [14C]‐methyl isocyanate, in its break‐seal container as received from the supplier, is reacted with the appropriate oxime or phenolic intermediate without exposing the isocyanate to atmospheric moisture. Results are presented for the insecticides carbaryl, carbofuran, aldicarb and methomyl demonstrating consistent yields of 85 to 95% using a procedure of uncommon sim