We have used spectroscopic ellipsometry to determine the Flory-Huggins parameters in thin films of polystyrene, poly(2vinyl pyridine), and poly(tert butyl methacrylate) swollen in controlled atmosphere of various common solvents. From the experimental data the concentration dependent Flory-Huggins interaction parameters between the homopolymers and the different solvents are revealed. Similar experiments were performed on binary copolymers of the three components. From the comparison between the degree of swelling of the homopolymers and the copolymers, respectively, the polymer/polymer Flory-Huggins interactions parameters are determined as a function of solvent content as well. (C) 2004 Elsevier Ltd. All rights reserved.