The thermal oxidation at 130 degrees C of polyoxymethylene homopolymer (H) and copolymer (C) samples of close initial molar masses has been studied by gravimetry, rheometry, X ray scattering, and tensile testing. Both samples undergo random chain scission and depolymerization. Their crystallinity ratio increases, whereas their long period decreases. All these changes are faster for H than for C. Tensile tests reveal that there is no significant change of behavior law except for the ultimate strain, which decreases abruptly when the weight average molar mass reaches a value of the order of 70 kg mol(-1) for H and 90 kg mol(-1) for C. At these molar masses values, the entanglement network of the amorphous phase has undergone only small damagement. In contrast, a chemi-crystallization process has induced significant morphological changes, especially a decrease of the interlamellar thickness. It is suggested that this latter phenomenon could be responsible for embrittlement. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 469-475, 2009