Mechanisms for the recently described reactions of isonitriles with carboxylic acids (Li, X.; Danishefsky, S. J. J. Am. Chem. Soc. 2008, 130, 5446) are explored with the B3LYP density functional method. The mechanism involves the formation of a carboxylate mixed formimidic anhydride intermediate via a concerted mechanism. This intermediate is then transformed to an N-formylamide by a concerted pseudopericyclic [1,3]-acyl shift. Mechanisms involving zwitterions or diradicals are discounted.