This work analyzes the influence of thermal degradation on the microstructure and the mechanical properties of low-density polyethylene subjected to aging at 70 degrees C in the dark for times up to 21 months. It is found that the polymer shows a gradual increase of its elastic modulus and a dramatic reduction of its ductility, due to secondary crystallization. Infrared spectroscopy (FTIR) reveals the autoaccelerated oxidation of the polymer after 5 months aging. It is observed that the unsaturated vinylidene groups initially present in the material are gradually overridden by vinyl groups and, eventually, by t-vinylene groups. Nuclear magnetic resonance (13C NMR) shows that the initial butyl chain branches are progressively completed by shorter ramifications, namely ethyl branches. These results are discussed in term of macromolecular mechanisms: (i) oxidation, (ii) chain scission, and (iii) crosslinking. (c) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011