Reactions of ethoxycarbonyl isothiocyanate (2a) with heterocyclic amines whose PKa values range from 2.95 to 9.17 were examined. Regardless of the base strengths, the heterocyclic amines except 4-aminopyridine afforded the addition products in a good yield in the nucleophilic reactions with 2a. 4-Aminopyridine (14) which afforded no addition product with 2a yielded ethyl N-(4-pyridyl) carbamate (16). Probably the nucleophilic substitution of 14 occurs at the ring nitrogen atom and the pyridinium salt (15) formed in the initial stage of the reaction rearranged intermolecularly and resulted in the formation of 16.