The polycondensation of poly(ethylene terephthalate) (PET) through ester interchange is a process strongly limited by mass transport effects. Therefore, design and scale up of industrial PET plants require a detailed knowledge of micro- and macro-kinetics. In this work, a comprehensive model for the polycondensation of PET has been developed taking into account (1) kinetic data, (2) equilibrium data, and (3) diffusion processes for the major by-products EG and water in the molten PET. The model was validated using own experimental data determined by thermogravimetric analysis. As a sample, the so-called "PET-prepolymer" was used. The kinetic parameters for the ester interchange reaction, for the water formation through esterification as well as for the most significant side reactions were taken from the literature. The mass transport was described using a classical diffusion equation according to Fick's law and a modified Wilke-Chang diffusion model. Diffusion coefficients where then calculated as a function of temperature and degree of polycondensation to give a good explanation of the process phenomena.