Here we report the synthesis, mechanism of formation, characterization and thermal decomposition of new barium borohydride chlorides prepared by mechanochemistry and thermal treatment of MBH₄–BaCl₂, M = Li, Na or K in ratios 1 : 1 and 1 : 2. Initially, orthorhombic barium chloride, o-BaCl₂ transforms into o-Ba(BH₄)_xCl_2−x, x ∼ 0.15. Excess LiBH₄ leads to continued anion substitution and a phase transformation into hexagonal barium borohydride chloride h-Ba(BH₄)_xCl_2−x, which accommodates higher amounts of borohydride, possibly x ∼ 0.85 and resembles h-BaCl₂. Thus, two solid solutions are in equilibrium during mechano-chemical treatment of LiBH₄–BaCl₂ (1 : 1) whereas LiBH₄–BaCl₂ (2 : 1) converts to h-Ba(BH₄)_0.85Cl_1.15. Upon thermal treatment at T > ∼200 °C, h-Ba(BH₄)_0.85Cl_1.15 transforms into another orthorhombic barium borohydride chloride compound, o-Ba(BH₄)_0.85Cl_1.15, which is structurally similar to o-BaBr₂. The samples with M = Na and K have lower reactivity and form o-Ba(BH₄)_xCl_2−x, x ∼ 0.1 and a solid solution of sodium chloride dissolved in solid sodium borohydride, Na(BH₄)_1−xCl_x, x = 0.07. The new compounds and reaction mechanisms are investigated by in situ synchrotron radiation powder X-ray diffraction (SR-PXD), Fourier transform infrared spectroscopy (FT-IR) and simultaneous thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), mass spectroscopy (MS) and temperature programmed photographic analysis (TPPA).