The purpose of this investigation is to evaluate thermal lifetime of polyesterimide using Weibull statistics. Thermal aging was performed on twisted pair specimens of copper wire insulated by a polyesterimide layer. The study shows that breakdown voltage varies versus aging time. Its increase is allotted to a crosslinking inducing a diminution of mean-free path of charge carriers leading thereby to a mobility decrease, whereas its decrease is attributed to the viscosity diminution expressing a weakening of molecular bonds and a mobility increase of charge carriers. Shape parameter changes in function of aging time. Its increase is ascribed to an arrangement of the molecular structure, whereas its decrease is due to an augmentation in the size of defects. The thermal endurance graph is a straight line indicating that the degradation is governed by a first-order chemical reaction. Activation energy and temperature index were determined. The degradation is governed by the dissolution of copper into the polymer and accelerates its degradation occurring at the polyesterimide-copper interface. Oxygen can diffuse into the insulation and attack copper resulting in the formation of copper oxide. The degradation is caused by the scission of imide and ester bonds. The process is followed by a change in color and a presence of cracks. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2886-2892, 2011