Abstract: The invention concerns new catalyst systems for the synthesis of polyesters, for instance for the manufacture of polyethylene terephthalate and its copolyesters. The catalyst system according to the invention consists of an antimony or germanium compound, a heterogeneous catalyst component and an ester of phosphoric acid or of phosphorous acid as stabilizer. The polycondensation rate both in the liquid phase and in solid phase polycondensation (solid state) can be increased by 30-100 percent with the smallest additions of the heterogeneous component.