Process for thermally stabilizing a polymer containing residues of a Sn(II), Sb(III), Pb(II), Bi(III), Fe(II), Ti(II), Ti(III), Mn(II), Mn(III), or Ge(II)-containing catalyst by treating the polymer at a temperature above its melting temperature with a peroxide selected from the group consisting of ketone peroxides, hydroperoxides, peracids, hydrogen peroxide, and mixtures thereof, wherein said peroxide is used in an amount less than 0.2 wt % based on the weight of the polymer and wherein the molar ratio of peroxy functionalities from said peroxide (p) to metal (M) ranges from 1 to 100; said metal (M) being selected from the group consisting of Sn(II), Sb(III), Pb(II), Bi(III), Fe(II), Ti(II), Ti(III), Mn(II), Mn(III), and Ge(II). The invention further relates to a method for determining the metal residue content of a polymer.