The influence of sodium cations on the thermal behavior of three aliphatic linear polyesters, namely poly(ethylene adipate)diol, poly(butylene adipate)diol and poly(hexamethylene adipate)diol, the major constituents of polyesterurethanes, was studied by investigating the activation energy of the thermal degradation processes of polymer/sodium ions interaction products in comparison with that of parent polyester, using the Levi-Reich method.