Inverse gas chromatography (IGC) has been widely used to determine the Flory-Huggins parameter, chi, between two polymers using a series of solutes as probes. Many studies showed that interaction parameters were probe dependent. In recent studies, it was proposed that the difference in interaction energy between functional groups of solutes and solvent mixtures could lead to an apparent solvent solubility parameter different from the volume average of the components. An equation was derived to relate the probe dependency to the deviation of the solubility parameter. By plotting phi(2)phi 3RT(chi(23)/V-2) vs. the solubility parameter of solutes, a straight line could be obtained with a slope proportional to the deviation of the solvent solubility parameter. The plot was shown to yield negative slopes for miscible polymer blends. When there was an unfavorable interaction between two solvents, an opposite situation would be observed. In this study, the method was tested in two copolymer systems, and the copolymers were considered as mixtures of two homopolymers. The plots showed straight lines with positive slopes when chi(23) was positive. The technique was shown to apply to copolymer systems also. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 719-725, 2011