Many unique properties of ionic liquids (ILs) are closely related to the extent of dissociation (a) in solution. However, most of the existing models for ILs assume either full dissociation or full nondissociation regardless of mixture compositions. In this work, the dissociation of ILs (CA = C+ + A(-)) is described as a chemical reaction. Together with the predictive COSMO-SAC model, the dissociation constant can be determined based on the value of a in the pure state. Our results show that the predicted composition dependence of a is in good agreement with experiment over the entire concentration range. We further examine the prediction of a variety of thermodynamic properties and phase behaviors of IL solutions, covering high (infinite dilution activity coefficient of solvent), medium (vapor-liquid and liquid-liquid), and low (osmotic coefficient and mean ionic activity coefficient) IL concentrations (a total of 9857 data points). Our results show that the composition dependence of IL dissociation has a significant impact on the phase behaviors of IL solutions. The dominating factors for the solution nonideality changes from short-range ion-pair interactions at high IL concentrations to long-range ion ion interactions at low IL concentrations.