Thermal properties of mixtures of gamma-butyrolactone (BL) with 1-butyl-3-methyl-imidazolium tetrafluoroborate (BMIBF4) or hexafluorophosphate (BMIPF6) have been investigated from -120 degrees C to 400 degrees C by differential scanning calorimetry (DSC) and X-ray Diffraction (XRD). From the result of these experiments, the liquid-solid phase diagrams of these mixtures have been inferred. The phase diagrams obtained with the two anions are similar but differ strongly from those obtained for organic solvent mixtures. The apparition of a vitreous phase is observed when the mole fraction in ionic liquid is higher than a critical mole fraction (X-critic). From this value, the number of solvent molecules in the solvation shell of BMI+ ion has been evaluated. Thermal analysis of the mixtures at high temperature have been run and show that the thermal decomposition is strongly dependent on the nature of the anion and that BL-BMIBF4 mixtures are more stable than BL-BMIPF6 mixtures at high temperature. (c) 2005 Elsevier B.V. All rights reserved.