A crystallizer was built and a procedure developed to accurately measure the eutectic solubility lines where ice and salt coexist in equilibrium with the solution, for potential application of Eutectic Freeze Crystallization. The eutectic solubility lines of the ternary system NaHCO3-Na2CO3-H2O were determined experimentally and calculated with the extended UNIQUAC model. The extended UNIQUAC model describes the experimental data quite well. Anhydrous NaHCO3 and Na2CO3 center dot 10H(2)O were the only two types of crystals present in equilibrium with ice crystals in the ternary system. At the quadruple point NaHCO3 and Na2CO3 center dot 10H(2)O are in equilibrium with a solution of about 4.34 wt% of Na2CO3 and 4.77 wt% of NaHCO3 at -3.32 degrees C. The anhydrous NaHCO3 crystals were needle shaped with lengths between 5 and 10 mu m, that were agglomerated into particles of about 100-300 mu m, while the Na2CO3 center dot 10H(2)O crystals were hexagonally shaped with sizes between 100 and 500 mu m. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.