Although determining the chemical states of salts and ions is critical in numerous fields, such as elucidating biological functions and maintaining food quality, the current direct observation methods are insufficient. We propose a spectral analysis method of directly observing the phase transitions of NaCl solutions using the changes in the charge-transfer-to-solvent band and the absorption band representing the first electron transition (Ã ← X̃) of H2O. The intensities of these bands may be observed using attenuated total reflection far-ultraviolet spectroscopy. According to the well-known phase diagram of aqueous NaCl, we observe spectral changes during freezing-thawing and may spectroscopically detect the phase transitions from liquid to mixed liquid-solid and solid phases, including eutectic crystals, in addition to their coexistence curves.