Abstract The dissolution of sodium chloride and sodium borohydride into each other resulting in formation of solid solutions of composition Na(BH 4 ) 1− x Cl x is studied. The dissolution reaction is facilitated by two methods: ball milling or combination of ball milling and annealing at 300 °C for three days of NaBH 4 –NaCl samples in molar ratios of 0.5:0.5 and 0.75:0.25. The degree of dissolution is studied by Rietveld refinement of synchrotron radiation powder X-ray diffraction (SR-PXD) data. The results show that dissolution of 10 mol% NaCl into NaBH 4 , forming Na(BH 4 ) 0.9 Cl 0.1 , takes place during ball milling. A higher degree of dissolution of NaCl in NaBH 4 is obtained by annealing resulting in solid solutions containing up to 57 mol% NaCl, i.e. Na(BH 4 ) 0.43 Cl 0.57 . In addition, annealing results in dissolution of 10–20 mol% NaBH 4 into NaCl. The mechanism of the dissolution during annealing and the decomposition pathway of the solid solutions are studied by in situ SR-PXD. Furthermore, the stability upon hydrogen release and uptake were studied by Sieverts measurements.