Predicting activity coefficients in polymers for arbitrary solute and polymers is of general interest for packaging, environmental, and membrane applications. We examined the extensions of the proposed off-lattice approach [Gillet et al. Ind. Eng. Chem.Res. 2009, 2010] for random and block copolymers. Based on a comparison with an explicit representation of random copolymers, the principles of a mean field approximation using only the properties of homopolymers have been established. The approach was successfully validated against the Flory-Huggins coefficients collected by Fornasiero et al. [AIChE J. 2002] for 19 aromatic solutes in ethylene-vinyl acetate with acetylation rates varying from 33% to 100%. The role of substituents and isomerism is elucidated. Previously collected data and this study suggest that the real chemical affinity of substituted aromatic compounds for water could be strongly underestimated by previous oversimplified rules and that tailored methods, such as this one, would be preferable.