The solubility in the (C2H5)2NH-(C2H5)2NH . H2CO3-H2O system was studied at 25 and 40-degrees-C. It was shown that the only phase is diethylamine hydrogencarbonate, the crystallization range of which is reduced with increase of temperature from 25 to 40-degrees-C. The results from the investigations are important in theoretical and practical respects, since it was the first time that the crystallization range of diethylamine hydrogencarbonate in the amine process for the production of soda ash had been established.