Mutual solubility of NH4HCO3 and NaHCO3 in aqueous solutions over a temperature range 293 K to 323 K has been measured. On the basis of these results, a polytherm section for the NH4HCO3 + NaHCO3 + H2O system has been drawn. In addition, a correlation between the solution densities and both salt concentrations has been found.