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We report a structural and thermodynamic investigation of the phase behavior of Ga(OH,F)-MIL53, a gallium-based MOF having the MIL-53 topology containing 0.7 wt% fluorine bonded to the metal. Despite some small structural differences, especially for the hydrated form, the overall physical chemistry behavior of Ga(OH,F)-MIL-53 is very similar to standard fluorine free Ga-MIL-53 material. A combination of in situ X-ray diffraction, in situ Fourier transform infrared spectroscopy, differential scanning calorimetry and heat capacity measurements allowed to establish that Ga(OH,F)-MIL53 under vacuum (i.e. in the empty material) exhibits two stable phases: a nonporous narrow-pore (np) phase favored at low temperature and a large-pore (lp) phase favored at high temperature, accompanied by a huge hysteresis effect. Structure determination of the hydrated material Ga(OH,F)-MIL-53_np_H 2 O obtained after synthesis, activation and rehydration, was also performed. Density functional theory calculations show that it is not a stable structure of Ga(OH,F)-MIL53 in absence of adsorbed water molecules. Instead, this hydrated structure is a swollen variant of the np phase, where the flexible framework has expanded to accommodate water molecules.