The organic salt guanidinium mesylate [Gdm][CH3SO3] is a promising phase change material for thermal energy storage applications. It has melting point Tm = 208 °C, and accompanying latent heat of fusion Hf = 190 J/g , which is significantly better in comparison with guanidinium salts coupled with other anions. Preliminary studies suggest that the crystal structure of [Gdm][CH3SO3] demonstrates significant N-H…O or N-H…Cl hydrogen bonds between the [Gdm]+ cations and the anions. Hence our objective is to probe more deeply the crystal and liquid-state structures of a family of organic salts based on guanidinium cation, that determine their ideal properties, i.e. high heat of fusion, required for a phase change material.