The relationship between structure and properties of rotationally moulded polyethylene was studied using three different contents of antioxidant - 0.04% (standard), 0.1% and 1% - and two different mould atmospheres - air and nitrogen. The mechanical behaviour of the moulded parts was interpreted in terms of the structure of the polymer and the level of degradation at the inner surface of the mouldings. The degree of degradation was assessed using Fourier transform-infrared spectroscopy, fluorescence microscopy and melt-flow index measurements. The microstructure was observed using polarized light microscopy and scanning electron microscopy. The mechanical strength was evaluated by impact testing using an instrumented drop weight machine. The results showed that as the maximum temperature of the gas inside the moulding increases, the impact strength also increases, reaching a maxim um at about 225 degrees C. This build up in impact strength is related to the improved sintering of the plastic powder/melt with time and temperature. However, as is well known in the industry, the moulding conditions needed to achieve optimum properties are critical because a small amount of overheating causes the impact strength, for example, to decrease dramatically. This decrease is due to the degradation that occurs at the inner surface of the samples. This occurs because the surface is in contact with oxygen during processing, leading to oxidation reactions in the material, predominantly resulting in cross-linking. The cross-linked material is responsible for the low impact strength and for the brittle behaviour of samples moulded at higher temperatures. The morphology of these samples is not characterized by the polyethylene spherulitic texture, observed across the thickness of the samples moulded at lower temperatures. The morphology at the inside surface was modified and gave rise to a thin layer of very small and imperfect spherulites, which disappear when the heating is too severe. Next to it is a columnar-type structure made of bundles of parallel fibrils. The increase of antioxidant content and the use of a nitrogen atmosphere caused a delay of the degradation process, but did not prevent it. No significant mechanical strength improvements were observed in these conditions, but a broader processing window was available.