A model of copolymer solids hypothesizing inclusion of the comonomer units in the crystalline domain is applied to calculation of solid-liquid transitions. Such an inclusion model is compared to an exclusion model hypothesizing exclusion of the comonomer units from the crystalline domain. Both models are used with the copolymer SAFT equation of state(1) to represent the solid-liquid transition temperatures of polyethylene and poly(ethylene-co-octene-1) in propane, isooctane, and a mixture of isooctane + ethylene. The inclusion model is found to be more predictive and to have a correct high polymer-concentration limit, which is the melting temperature of pure polymer. Furthermore, the inclusion parameters, the fold energy and the energy penalty, are found to be solvent-and composition-independent, while the exclusion parameter, the crystallizability, is found to be both solvent- and composition-dependent.