Poly (4-methyl styrene – alt – maleic anhydride) (PMS-A) copolymer was synthesized by free radical polymerization, and then modified with n-propyl amine through ring opening of maleic anhydride units to prepare modified poly (4-methyl styrene – alt – n-propyl maleimide) (PMS-I) copolymer. FT-IR analysis confirmed the composition of these copolymers. Thermogravimetric analysis (TGA), glass transition temperature (Tg), and gel permeation chromatography (GPC) were investigated for the copolymers. Some thermodynamic and physicochemical properties were determined for different groups of test solutes at infinite dilution by inverse gas chromatography (IGC) in the temperature ranges 373–393 K and 373–433 K for the PMS-A and the PMS-I copolymers, respectively. The physicochemical characterization of the IGC included the determination of the molar heat enthalpy of sorption, mixing and vaporization (ΔHS 1, ΔH∞1 , ΔHV), weight fraction activity coefficient, Ω∞1 , Flory–Huggins interaction parameters, X∞12 at infinite dilution, Hildebrand solubility parameter, δ2, total and partial Hansen solubility parameters, δT, δd, δp, δh. The results showed that the solubility of the PMS-A copolymer in nonpolar solutes is poor but moderate for the PMS-I copolymer. The solubility of the PMS-A copolymer in benzene and toluene solutes is moderate, but it is good for the PMS-I copolymer. The solubility of the PMS-A copolymer in ketones, halogens, nitriles, acetates, heterocyclic, and alcohol solutes is good, but the solubility of the PMS-I copolymer with these solutes is the best. Values of the Hildebrand solubility parameters of the PMS-A and PMS-I copolymers ranged from 17.57 – 16.73 and 19.48 – 18.06, respectively. In addition, the total Hansen solubility parameters ranged from 18.82 – 17.12 and 16.65 – 15.20.