The behaviour in the title reaction of ten primary amines, with a basic pKa range of 11 units, has been studied. Uncatalysed nucleophilic attack of free base amine and competing uncatalysed hydrolysis are the only reactions found except in the case of hydrazine, where an additional term is attributed to nucleophilic attack of the hydrazinium cation. The Bronsted plot shows that the sensitivity of methyl chloroformate to the basicity of the nucleophile decreases with increase in the latter, and the significance of this is discussed.