In this article new thermodynamic descriptions of the (LiF + UF3) and the (NaF + UF3) systems based on new calorimetric data are presented. The experimental investigations were carried out using differential scanning calorimetry (DSC) to obtain phase transition temperatures between (0 and 60) mol% of UF3. On the basis of these measurements a thermodynamic assessment of the (LixU1-x)F3-2x and (NaxU1-x)F3-2x liquid solutions, using the quasi-chemical model was achieved. UF3 undergoes a disproportionation at higher temperatures to UF4 and U metal which had a significant impact on the transition temperatures in the (LiF + UF3) system. For this reason the influence of the presence of up to 15 mol% of UF4 in this system was calculated and compared to the experimental results. The final thermodynamic description was corrected for the pure binary system. (C) 2012 Elsevier Ltd. All rights reserved.