An equation describing solubilization in aqueous systems by cosolvents was developed by treating a mixed solvent as a linear combination of its components. This equation can successfully explain both the exponential increases and the exponential decreases in aqueous solubility that are frequently observed with the addition of cosolvent. It also provides a means of estimating to what extent a particular drug can be solubilized and how much cosolvent would be required to accomplish a particular degree of solubilization.