The generation behavior and the molecular structure of a heat-stable salt arising from coexistence between formic acid and an amine component contained in a CO2 solvent have been studied. The heat-stable salt is thought to be generated in a reversible endothermic reaction and reach reaction equilibrium in the amine solution. The heat-stable salt has been proven to be formamide of the amine that is believed to be generated in a dehydration reaction of ammonium formate.