The melting (crystallization), separation, and boiling points of a mixture of propylene oxide and water were determined and used to plot the phase diagram of the mixture. The composition of propylene oxide crystal hydrate was investigated. The triangular diagram of separation of the system at 20°C was plotted with the results of studying the mutual solubility in the ternary straight-run naphtha -propylene oxide -water system. Key words: ternary straight-run naphtha-propylene oxide-water system, solubility in ternary system, propylene oxide crystal hydrates.Due to the prohibition of use of organometallic compounds [1], oxygenates are the most important octaneboosting additives to motor fuels: ethanol, isopropanol, tert-butanol, isobutanol, C 5 ethers, etc. Propylene oxide (PO) has been examined as another oxygenate [2][3][4]. The mechanism of action of oxygenates, including propylene oxide, was proposed in [5]. The physicochemical and performance characteristics of composite fuels must be investigated for incorporating oxygenate compositions with propylene oxide.We investigated the mutual solubility in the straight-run naphtha-propylene oxide-water (SN-PO-water) ternary system and plotted the solubility diagrams. In the first stage, we studied the PO-water binary mixture. The melting (crystallization), and boiling (GOST 18995.5-73) points, and separation point of the mixture were determined [6]. The PO content in the binary and ternary mixtures were determined by a chemical method based on the reaction with HCl [7].The melting (crystallization, separation, and boiling points of the PO-water mixture coincide. The phase diagram of the system was plotted for better understanding of the results obtained (Fig. 1). The region of separation of this mixture can arbitrarily be divided into three parts. In the first and third parts, the separation and melting (crystallization) points coincide, while they differ in the second part, but the boiling point coincides with the separation point in this part. It is characteristic for the melting point of the PO-water mixture to decrease sharply in the third part for a PO content above 95 wt. %.The phase diagram shows that sediment stably precipitates in a wide range of PO contents at -6.5°C. This sediment is a white, crystalline substance which was separated on a filter at the temperature of -10 to -21°C. The melting points