Abstract: The invention pertains to new catalyst systems for polycondensation reactions, for example for producing polyethylene terephthalate. In accordance with the invention, complex compounds with hydrotalcite-analogous structures of general formula [M(II)1−xM(III)x(OH)2]x+(An−x/n).mH2O are used, wherein M(II) represents divalent metals, preferably Mg or Zn or NI or Cu or Fe(II) or Co, and M(III) represents trivalent metals, for example Al or Fe(III), and A represents anions, preferably carbonates or borates. These catalysts can be calcinated and can be used in combination with phosphorus compounds that contain at least one hydrolyzable phosphorus-oxygen bond.