α‐Sulfonated fatty acid methyl ester salts (MES), synthesized from renewable plant resources, are an example of green surfactants used in eco‐friendly washing detergents because of their excellent detergent properties, biodegradability, and enzyme stability. Although various physicochemical properties of MES crystals and micelles have been studied, mixed systems composed of MES and other surfactants have not been well studied. We investigated the crystalline structures of hydrated solids in mixed systems containing MES and soaps, which have been utilized as detergents, using small‐ and wide‐angle X‐ray scattering (SWAXS), differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy techniques. The minimum dissolution temperature, determined by visual observation, of a 4:1 M ratio of the sodium salt of α‐sulfonated palmitic acid methyl ester (C16MES‐Na) and sodium palmitate (C16‐Na), is indicative of a eutectic mixture. SWAXS measurements reveal that C16MES‐Na and C16‐Na crystals are formed separately in this system. Eutectic mixtures were also observed for the C16MES‐Na/C16MES‐K (α‐sulfonated palmitic acid methyl ester potassium salt) system and in the C16MES‐K/C16‐Na system. Furthermore, in addition to C16MES‐K and C16‐Na crystals, C16MES‐Na crystals were also formed in the C16MES‐K/C16‐Na system, through counterion exchange during crystallization.