Mutual solubility and tie line data at 25 degrees C, and atmospheric pressure are presented for the quaternary system water-acetic acid-2-butanone-cyclohexane. The system contains two pairs of partly miscible compounds with very different solubilities which produce a solubility surface with a large hump and tie lines which do not lie on tie line planes. The UNIQUAC equation is used to correlate the liquid-liquid equilibrium data.