Fluorine substitution in CaH2 has been studied by means of experimental and theoretical methods. Samples with various compositions have been prepared by ball milling. In situ X-ray diffraction analysis has been carried out as a function of temperature by synchrotron radiation experiments. An increase of mixing has been observed during heating, suggesting that mixing is thermodynamically favoured but it is kinetically hindered at low temperatures. Ab initio DFT calculations have been performed to estimate the thermodynamic mixing properties of both orthorhombic and cubic solid solutions. On the basis of ab initio results and literature information, a thermodynamic assessment within the CALPHAD framework has been performed and the pseudo binary CaH2-CaF2 phase diagram has been calculated. The formation of orthorhombic and cubic terminal solid solutions in the CaH2-CaF2 system is predicted, in good agreement with experimental findings.