The solubility of urea in mixtures of acetonitrile (MeCN) and water was measured in terms of mole fraction over the entire range of MeCN/water compositions. With respect to mole fractions of the three components, it was found that in water-rich mixtures the mole fraction of urea in urea saturated solutions showed almost no decline even after 25 mole percent water was replaced with MeCN. in the MeCN range of (77 to 86) % (v/v), the mixtures divided into two liquid phases after urea saturation. The more dense layer contained a roughly 2:2:1 mole ratio of water/MeCN/urea, while the less dense layer was primarily MeCN with small amounts of water and urea. Changing the MeCN content within the (77 to 86) % (v/v) range caused the volume ratio of the two layers to vary, but the composition of the upper and lower layers remained essentially constant. This behavior suggests that the water-rich phase is saturated in both urea and MeCN. The driving force for the phase separation is explained as the tendency for urea to promote self-association of the individual water and MeCN components. In MeCN-rich mixtures close to the limit of pure MeCN, urea solubility varied in a roughly linear fashion with water content. This behavior is explained qualitatively by the ability of water to act as a hydrogen-bond donor to urea, an interaction which is not possible with a nonprotic solvent such as MeCN.