The solubility of hydrophobic organic chemicals (HOCs) in partially-miscible solvent mixtures was investigated. In agreement with previous findings, it was observed that there is a limited domain in which nonideality effects are important; appreciable concentrations of partially-miscible (with water) organic solvents are required in order to observe significant effects on solubility. Deviations from the log-linear cosolvency model were attributed to solvent-solvent interactions, and hence quantified by the degree of nonideality of the solvent mixture. An expression was proposed for estimating the deviations from the log-linear model. For each solvent component, a nonideality term closely related to its partial excess free energy of mixing was added to the log-linear model. Predictions of solubility in binary and ternary solvent mixtures were in good agreement with experimental data. Deviations from the log-linear model were attributed to solvent nonideality which implies that there is no fundamental or mechanistic difference between the cosolvencies of partially- and completely- miscible solvents. The observed differences in cosolvencies are only in magnitude, which results from a combination of the degree of nonideality of the mixture (activity coefficients) and concentrations of the components. Examples of the application of the proposed expression are presented.