The phase diagram of the alpha-sulfonated palmitic acid methyl ester sodium salt (SFMe)-water system was studied by measuring the phase transition temperatures (T-c) and the X-ray diffraction patterns for each solid phase of SFMe at various water contents. In the developed phase diagram, one anhydrous solid phase and three distinct hydrated solid phases (SFMe . 2H(2)O, SFMe . 5H(2)O, and SFMe . 10H(2)O) were recognized as well as liquid crystalline phases, a micellar phase, and a monomer phase. From the analysis of the T-c changes due to the water contents of each SFMe solid phase according to the Flory-Huggins-Scott equation, it was found that these hydrated SFMe solids form eutectic mixtures with each other. When the dry and wet SFMe solids were left in an atmosphere with ordinary relative humidity and temperature, the water content of the SFMe solid converged into the water content corresponding to that of the SFMe . 2H(2)O.