Polyoxymethylene (POM) copolymer and its blends with poly(ethylene-methylacrylate) (EMA) or poly(styrene-butadiene-styrene) (SBS) up to 5 wt % were conducted to thermooxidative ageing in oven at 140 degrees C for 111 days. POM showed continued degradation as seen from the gradual decrease in the crystallization temperature from differential scanning calorimetry (DSC) study, while no change in Fourier transform infrared spectra (FTIR) and low weight loss were observed (2.5% after 111 days' ageing). The POM degradation was characterized by the initial increase of crystallinity due to crystal perfection, which then kept nearly unchanged until 35 days' ageing, and lastly increased again to result in embrittlement and decrease in tensile strength. Increase in the tensile stress and strain was observed up to 35 days' ageing. POM blends with SBS or EMA had similar degradation behavior as POM, but addition of SBS accelerated the POM degradation significantly, while POM blend with 1 or 3% EMA just showed slightly lower thermal stability than POM. Degradation in POM and SBS/POM occurred in amorphous phase while EMA/POM degraded in both amorphous and crystal phase. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 376-388, 2011