Samples crystallized in the system l-Val—l-Ile—water by cooling and isothermal methods were studied. The ternary solubility diagram of the system was established based on liquid chromatography data. A non-equimolar 2:1 discrete compound V2I was discovered, and its crystal structure was determined via single crystal diffractometry. Narrow regions of solid solutions close to the discrete phases and two wide two-phase areas were distinguished by X-ray diffractometry analyses.