Although poly(ethylene terephthalate) (PET) has excellent basic properties, this polymer tends to crystallize rapidly and has a rather high melting temperature, a low glass-transition temperature, and low impact on notched articles for some potential applications. Copolymerization is a reasonable method for improving the properties of PET. 1,4-cyclohexane dimethanol (CHDM) is one of the most important comonomers for PET. In this research, PET and PET copolymers containing 5-30% CHDM were prepared from comonomer mixtures by two-step melt polycondensation. The copolymers were synthesized in a homemade laboratory setup. The first synthesis step was conducted under pressure, and the second was performed in vacuo at a high temperature (230-290 C). The microstructure of the synthesized copolymers was studied with Fourier transform infrared and nuclear magnetic resonance. The comonomer content in the polymer chain was determined from the nuclear magnetic resonance spectrum. The presence of the comonomer in the copolymer chain was random. Differential scanning calorimetry was used to study the thermal properties of the copolymers to detect changes in the polymer properties. CHDM reduced the heat of fusion and melting and glass-transition temperatures of the PET copolymers. Process modeling was performed with mass balances of different functional groups and species. Equations of mass balances were integrated numerically. Numerical simulation and experimental results were in very good agreement. By modeling, the effects of the reaction temperature and feed molar ratio on the conversion and formation of diethylene glycol were studied.