Expressions based on Regular Solution Theory are evaluated as a means of predicting solubilities in lipids and lipid/water partition coefficients, and confirming observed experimental trends of these parameters. Employing chlorobenzenes and chlorinated dibenzo‐p‐dioxins as solutes, and triolein as a representative lipid, it is shown that triolein/water partition coefficients (Ktw) can be expressed as a function of the solute Hildebrand parameter and molar volume. There is relatively little difference in calculated values of Ktw when a Flory‐Huggins modification is incorporated into the theoretical approach to accommodate a molecular size disparity between solutes and triolein. Results confirm that the relationship between log Ktw and the logarithm of the octan‐1‐ol/water partition coefficient (log KowJ is slightly curvilinear due to increasing solute non‐ideality in triolein, demonstrating the utility of this theoretical approach. Regular Solution Theory is also shown to be useful for predicting solubilities in lipids such as triolein. Liquid and supercooled liquid solubility of chlorobenzenes and chlorinated dibenzo‐p‐dioxins in triolein decrease with solute size, molar volume or log Kow value. When supercooled data is converted into lipid solubility of solid material, the decrease is even more pronounced. These trends alone, however, cannot account for observed decreasing bioconcentration factors with relatively large, hydrophobic solutes.