The phase behaviour and interaction parameters of binary blends of copolymers involving cyclohexyl methacrylate (CHMA), methyl methacrylate (MMA) and various styrene derivatives (styrene (S), alpha-methylstyrene (AMS) and p-methylstyrene (PMS)) are examined using the lattice fluid theory of Sanchez and Lacombe. ＇Bare＇ interaction parameters (Delta P-ij*) for various monomer unit pairs were deduced where possible from lower critical solution temperature (LCST) type phase boundaries and from a binary interaction model. The calculations reveal that values of the interaction parameters are less favourable to miscibility when PMS is employed instead of S in blends with CHMA and MMA. These data are in good agreement with the experimental results and the interaction parameters obtained are translatable from one system to another. The results are discussed in terms of interactions between monomer unit pairs, equation-of-state effects and thermodynamical properties. (C) 2000 Elsevier Science Ltd. All rights reserved.