Thermodynamic properties underlying the shift in solute retention that results from a change in composition of a binary mobile-phase fluid in supercritical fluid chromatography (SFC) are identified and discussed. It is shown that the composition change in the mobile-phase fluid affects the retention properties of the stationary phase even when interfacial adsorption is neglected, and bulk partitioning is supposed to be the only mechanism of solute retention. In the resultant relationship, the effects of modifier (cosolvent) on density are explicitly separated from the effects on intermolecular interactions. The relative contributions of the separate terms to the observed shift in retention are partly illustrated with the literature experimental data available at the moment. It appears that, in general, several corrections should be applied when using SFC to probe cosolvent effect in a solute-supercritical fluid mixture but that the corrections become less important as the cosolvent effect increases.