Vapor–liquid equilibrium data were determined for five polymer + toluene systems at isothermal conditions between 333.15 and 373.15 K. Polymers comprise copolymers and terpolymers of octadecyl acrylate (ODA), acrylic acid (AA), styrene (St), and 1-vinyl-2-pyrrolidone (VP) because of their practical importance as flow improvers for crude oil and/or derivatives. The need to measure these systems has emerged because relevant phase equilibrium data are not available in literature. All-glass micro-ebulliometer with circulation of liquid phase was used for measurement of total pressure over polymer + toluene mixtures, as described in our earlier study.1 To analyze the obtained data, we opted for the prediction of phase behavior, as the data of two experimental points, including concentration end points, could not be reduced with use of the UNIQUAC equation, as is e.g. in the Polymer Solution Data Collection by Hao et al.2 We used two predictive models, the Entropic-FV activity coefficient model3 and the GC-Flory EOS model, 4 to estimate the activity of toluene in a mixture with a polymer. Both models are based on the group contribution method. Two terpolymers, namely poly(ODA0.79–AA0.11–VP0.10) and poly(ODA0.82–St0.05–AA0.13) in mixtures with toluene were chosen as examples of solvent activity predictions, because values of all necessary group parameters for both models were at hand. Figures 1 and 2 show the prediction of toluene activities in both the terpolymer solutions, respectively. It is obvious that the models are mutually comparable and in a good agreement. Moreover, the dependence of solvent activity on concentration provides a qualitative description of particular system behavior over the whole concentration range including activity trends, since the prediction is based on group contributions, which comprises the structure of components involved. It is necessary to point out, that prediction procedures were not used for validation of experimental data, but to give an idea about the trend in activity vs. concentration dependence. As it can be seen, good agreement with experimental data was achieved. Figure 1. Activity of toluene in poly(ODA0.82–St0.05–AA0.13) at 363.15 Figure 2. Activity of toluene in poly(ODA0.79–AA0.11–VP0.10) at 353.15 K. References: 1. J. Pavlíček, G. Bogdanić and I. Wichterle, Fluid Phase Equilib., 2010, 297 (1), 142–148. 2. W. Hao, H. S. Elbro and P. Alessi, Polymer Solution Data Collection. 1: Vapor–liquid Equilibrium, Chemistry Data Series XVI, Part 1, DECHEMA: Frankfurt/M., 1992. 3. G. M. Kontogeorgis, Aa. Fredenslund and D. P. Tassios, Ind. Eng. Chem. Res., 1993, 32 (2), 362–372. 4.G. Bogdanić and Aa. Fredenslund, Ind. Eng. Chem. Res. 1994, 33 (5), 1331–1340.