In search of a high-capacity hydrogen storage system, we have investigated the thermodynamic properties of borohydrides [M(BH₄)_n]. Using positive muon-spin rotation and relaxation (μ⁺SR), we have acquired experimental data for a wide range of different powder samples below ambient temperature. Zero-field μ⁺SR measurements indicate the formation of the H-μ⁺-H system in LiBH₄, NaBH₄, KBH₄ and Ca(BH₄)₂, but not in Mg(BH₄)₂. It is also found that the amplitude of the HμH signal (A_HμH) varies with the electronegativity (χ_P) of Mⁿ⁺. This is because, when χ_P of Mⁿ⁺ is small, [BH₄]^- should be more negative, resulting in an increase in the electron-density of H^- ions. Therefore, A_HμH increases with decreasing χ_P. Since the thermodynamic stability of M(BH₄)_n also depends on χ_P, A_HμH is thought to be a microscopic indicator for the stability of borohydrides.