The use of aqueous two-phase extraction can be attractive for some separations, but the choice of such systems is frequently done in an empirical manner. Predictive models are needed for optimal design, but previous ones appear to have limitations. In addition, these systems have usually been used previously at temperatures around 25 degrees C; however, it is possible that better separations may be achieved at other temperatures. An equation is developed here which can predict partitioning for a wide range of solutes. The effect of temperature on partitioning has been determined, and this can also be predicted by using the equation. Thus, this model may prove quite useful in designing and optimizing extractive separations.