On the basis of the Van Deemter model of the chromatographic process, the infinite dilution diffusion coefficients of dichloromethane, chloroform, and carbon tetrachloride in poly(ethylene terephthalate) (PET) have been measured over a wide range of temperatures from 373.15 to 413.15 K by inverse gas chromatography. The relationships between the infinite dilution diffusion coefficients and molecular size of the solvent, as well as the temperature, were investigated. Meanwhile, the measured diffusion data were compared with the theoretical predictions by free-volume theory. The results indicated that neglecting the molecular interactions, i.e., the diffusivity energy term in the free-volume equation, may result in a poor prediction of the diffusion coefficient for PET-chlorinated methane systems because of the strong molecular interactions. The good agreement between the experimental data and the theoretical predictions by the proposed method showed that it is reasonable to take the diffusivity energy term into consideration for those systems with strong molecular interactions, and, furthermore, the infinite dilution diffusion coefficients at different temperature can be predicted accurately with this method.