Physical ageing of polymers is a well-known phenomenon reflected in structural, physical and mechanical property changes while the polymer gradually approaches its state of equilibrium [1]. Crystallizable thermoplastics exhibit upon ageing changes in properties the extent of which mainly depends on the polymer's thermal and mechanical history. Usually, physical ageing in crystallizable polymers has been related to structural changes such as volume relaxation and secondary crystallization. Coxon and White [2] have recently reported on another ageing effect related to partial relaxation of residual stresses in injection-moulded polypropylene. The stress levels were found to be markedly reduced upon ageing at a temperature as low as 4 0 ° C (below Tg). Furthermore, they have suggested that in semicrystalline polymers ageing may proceed even after annealing at elevated temperatures. Polyoxymethylene (POM), an engineering semicrystalline thermoplasic, exhibits a relatively high degree of crystallinity. Its structure, morphology and mechanical behaviour are sensitive to mechanical and thermal history [3-5], which is especially pronounced in injection-moulded articles. In addition, POM is expected to exhibit post-moulding warpage, dimensional changes and even cracking, the extent of which strongly depends on moulding process parameters such as melt and mould temperature and injection rate and pressures. Hammer et al. [6] have found that unstable low levels of crystallinity produced by quenching of POM increase rapidly on exposure to room temperature which is high above the polymer Tg [7]. Moreover, quenching of a polymer from elevated temperature results in the build up of residual stresses (e.g. [8, 9]), the level of which depends on the initial and final temperatures, the crystallization and glass transition temperatures and the Blot number, which takes into account the specimens dimensions, thermal diffusivity and heat transfer coefficient. These residual stresses may be an additional source for the observed changes occurring on ageing of POM [3, 5]. The objective of the present work is to investigate simultaneously the room-temperature ageing effect on the residual stresses profile and the degree of crystallinity in a quenched POM. Polyoxymethylene (Delrin 150, Dupont) granules were injection moulded to form 110mm × 110mm x 5 mm plates. The plates were first annealed at elevated temperature to relax all stresses and then slowly cooled to room temperature. Bars 10mm × 110ram in dimension were cut from the plates and were placed in an aluminium mould to avoid any distortions during the quenching experiments. The mould was constructed with six rectangular cavities, to accommodate the POM bars. Following the insertion of dry bars, the mould was tightly closed and then heated in an oven to 180°C (> Tin). Quenching was carried out by immersion of the mould into an ice-water bath. The residence time in both oven and bath was sufficient to attain a uniform desired temperature through the bars thickness. The quenched bars were demoulded and left to age at room temperature up to 150 days. Residual stresses were measured by the "layer removal" method, described by Treuting and Read [10]. The experimental procedure was described elsewhere [9]. The same "layer removal" method was also utilized to measure the through-the-thickness degree of crystallinity gradient in the POM bars. Per cent crystallinity was determined using wide-angle X-ray diffraction in the reflection mode. To verify the applicability of this procedure the crystallinity gradient in annealed bars was determined. A constant level of 82 + 2% crystallinity was obtained, which is satisfactorily within the accuracy limits of the X-ray degree of crystallinity determination. Residual stress profiles (stress against distance from the specimen's centre) of POM specimens having an initial uniform temperature profile of 180°C and quenched to ice-water temperature followed by ageing at room temperature are depicted in Fig. 1. The quenching of a melt to a temperature below the crystallization temperature resulted in the build up of compressive stresses in the surface layers and tensile stresses in the inner layers, as predicted by the thermoelastic theory [11] and previously observed in amorphous (e.g. [9]) and semicrystalline polymers [2, 12]. However, the profiles include unbalanced compressive and tensile stresses. Upon ageing the surface compressive stresses first increase to rather high values, attaining a maximum value of about 50 MN m -~, after 120 days at room temperature, followed by a large decline upon further annealing. Simultaneously, the tensile stresses at the centre increased with ageing; however, the changes measured are quite small, between 5 and 15MNm -2. It should be mentioned that all residual stresses were calculated, for practical reasons, on the basis of a uniform and constant elastic modulus, which is not the case in any quenched polymer [13], especially not in the presently studied system. Because the modulus value was taken as that of a quenched specimen the reported residual stresses (Fig. 1) are actually underestimated. The application of the "layer removal" method for