Disclosed is a method for the preparation of a polyester, which method comprises in a first step, reacting a dicarboxylic acid or a C1-C4 dicarboxylic diester with a diol at a suitable temperature and pressure to effect esterification or transesterification to prepare a precondensate and in a second step, reacting the precondensate to effect polycondensation at a suitable temperature and pressure to prepare a high molecular weight polyester, where a metal phosphonic acid complex compound of the formula is employed in the first step, in the second step or in both the first and second steps as a reaction catalyst; and wherein n is 0, 1, 2, 3, 4, 5 or 6; p is an integer from 1 to 30; M is a metal selected from the group consisting of Li, Na, K, Cs, Be, Ca, Mg, Sr, Ba, Al, Sb, Cd, Mn, Fe, Co, Ni, Cu and Zn; v is the valency of the metal M and is 1, 2 or 3; q is an integer from 1 to 20 where q=2p/v; and R1 is hydrogen, straight or branched chain alkyl of 1 to 36 carbon atoms, straight or branched chain alkenyl of 1 to 36 carbon atoms, cycloalkyl of 5 to 8 carbon atoms or aryl of 6 to 10 carbon atoms; or is said alkyl, alkenyl, cycloalkyl or aryl substituted by one to four hydroxyl, halogen, straight or branched chain alkoxy of 1 to 8 carbon atoms, straight or branched chain alkyl of 1 to 8 carbon atoms or amino groups. The metal complex catalyst compound is in particular