Vapor pressure measurements were performed for solutions of poly(methyl methacrylate-ran-tert-butyl methacrylate) with different weight fractions of tert-butyl methacrylate units, and their parental homopolymers in chloroform at 323 K, over a large domain of concentrations. The Flory-Huggins interaction parameters obtained from these experimental investigations show complex dependences of the Flory-Huggins interaction parameter on concentration and copolymer composition. This behavior can be modeled by taking into account an approach which considers the ability of the polymers to rearrange in a response to changes in their molecular surroundings [Adv. Polym. Sri. 2011, 238, 1-66]. According to this concept, the mixing process is subdivided into two clearly separable steps and accounts for the specific interactions between the solvent and copolymer segments.