The prediction of 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-DiMazio criterion. The model assumes zero entropy at the glass transition temperature and treats molecules as semi-flexible chains. This stiffness is associated with a flex energy. Whereas the application of the model is straight forward, a new formalism using the dyad concept was developed for copolymers. The preliminary results we obtained are consistent with experimental for some copolymers. For polymer solutions, the model appears to be only qualitative.