Abstract: A combined proportioning valve and mixer usable in a liquid chromatography system for mixing solvents in predetermined ratios, includes a magnetically driven stirrer rotatably disposed in a central chamber. Plural inlet lines allow solvents to enter the chamber at spaced positions. The chamber also communicates with outlet lines and is provided with means for preventing air bubbles from being entrapped in the stirrer and blocking flow through the system. Selectively controlled valve members control the flow of solvents through the inlet lines so as to achieve the preselected ratio of solvent mixing.