Melting temperatures and liquid-liquid equilibrium temperatures have been measured, and the liquid-liquid, solid-liquid phase diagram was constructed for xCH~3CN+(1-x)H~2O. It is a simple eutectic system with the liquid separation. Eutectic temperature and composition are 227.44K and x=0.955, respectively. The upper critical temperature is 271.0K, the critical composition is x=0.35, the critical exponent n=2.64. The equilibrium temperature of two liquid phases with ice is 263.07K. Excess molar Gibbs free energies were calculated at 263.07K, the largest value of G~m^E is 1174J/mol when system separates into two liquid phases.

Key words: PHASE EQUILIBRIUM, LIQUID-LIQUID EQUILIBRIUM, ACETONITRILE, PHASE DIAGRAM, GIBBS FREE ENERGY, SOLID-LIQUID EQULIBRIUM