We have developed a new model to describe the melting point depression of isomorphic copolymers. The model combines the common models applied for nonisomorphic and isomorphic copolymers and is capable of fitting the transition between both assuming the concentration dependence of the mean defect Gibbs energy. The model is tested with success on a set of melting temperatures of copolymers from ethylene terephthalate and ethylene 2,6-dicarboxynaphthanoate (PET and PEN), demonstrating the influence of defect aggregation during copolymer crystallization. The comparison of the model fits to the free energy values for comonomer inclusions from thermodynamic-integration simulations, making obvious the power of that computational method.