Experimental data are presented to show the points of incipient hydrate formation in the methane-propylene-water system. The addition of 1.4% propylene to methane lowered the equilibrium pressure 400 lb./sq. in. at 50°F. The locus of the four-phase equilibrium consisting of hydrate, vapor, water-rich liquid, and hydrocarbon-rich liquid was determined. This was terminated by the appearance of a critical condition at 1,370 lb./sq. in. abs. and 69.3°F., where the vapor phase contained 34.8% propylene on a dry basis. Below this pressure the hydrate, vapor, water-rich liquid equilibrium does not exist for solutions containing more than about 25% propylene. Solid-vapor equilibrium ratios were estimated for propylene hydrates. The significant features of the phase diagrams for the system are discussed in some detail.