Abstract: The invention relates to a process for making a modified thermoplastic polyester by reacting a thermoplastic polyester with a nitrogen-containing compound, wherein the thermoplastic polyester is first contacted in the solid state with an inert gas atmosphere containing at least one amino-compound of formula H2N—R—(X)n, wherein n is an integer?0, R=a linear, branched or cyclo-aliphatic or aromatic residue comprising 1–24 C-atoms and optionally containing hetero atoms, X?—NH2 or another functional group reactive with the polyester, at a temperature T(a) between room temperature and the melting point Tm of the polyester, during at least 1 s; and then heated to a temperature T(b) between T(a) and Tm, during up to 10 hours; and thereafter solid state polymerized at a temperature T(c) between (Tm?50) and Tm, during up to 100 hours, where T(a)<T(b)<T(c).