The glass transition temperature of polymers and polymer solutions was approached through a combination of the group-contribution, lattice-fluid equation of state and the Gibbs-DiMarzio criterion. The model assumes zero entropy at the glass transition temperature and treats molecules as semiflexible chains. This stiffness is associated with a flex energy obtained from the glass transition temperature at atmospheric pressure. Whereas the application of the model is straightforward for homopolymers and polymer solutions, a new formalism using the dyad concept was developed for copolymers. It takes into account the copoly-mer composition as well as the sequencing of the monomers. The results obtained are consistent with experimental data. For polymer solutions, the model predictions are semiquantitative depending on the system. The interaction parameter required for binary systems was found to have little effect on the glass transition temperature predictions. (C) 2003 Wiley Periodicals, Inc.