Chemical reactions of hydrogen storage materials often involve mass transport through a bulk solid. Diffusion in crystalline solids proceeds by means of lattice defects. Using density functional theory (DFT) calculations, we identify the stability and the mobility of the most prominent lattice defects in the hydrogen storage material NaBH4. At experimental dehydrogenation conditions, the Schottky defects of missing Na+ and BH4– ions form the main vehicle for mass transport in NaBH4. Substituting a BH4– by a H– ion yields the most stable defect, locally converting NaBH4 into NaH. Such a substitution most likely occurs at the surface of NaBH4, releasing BH3. Adding Mg or MgH2 to NaBH4 promotes this scenario.