In the batch polymerization of polyethylenterephthalate, the main reaction is an equilibrium-reaction. The removal of ethylenglycol from the melt determines the velocity of the polycondensation process, because it shifts the equilibrium to the side of chain growth. In this paper, we report about the implementation of an observer, which is based on a polymerization model in a pilot plant reactor. The observer is able to determine two important concentrations in the polymer melt as well as the product of mass transfer coefficient and specific surface. The knowledge of the latter parameter offers new possibilities for improved process control.