In this study, 65 wt% aligned untreated long hemp fibre/epoxy (AUL) and aligned alkali treated long hemp fibre/epoxy (AAL) composites cured at 70 degrees C using compression moulding were subjected to accelerated weathering using an accelerated weathering chamber with UV-irradiation and water spray at 50 degrees C for four different time periods (250, 500, 750 and 1000 h). After accelerated weathering, tensile strength (TS), flexural strength, Young's modulus (YM), flexural modulus and fracture toughness (K(Ic)) were found to decrease and impact energy (IE) was found to increase for both AUL and AAL composites. AUL composite had greater overall reduction in mechanical properties than that for AAL composite upon exposure to accelerated weathering environment. FTIR, TGA and WAXRD analyses of the accelerated weathered composites support the results of the deterioration of mechanical properties upon exposure to accelerated weathering environment. (C) Koninklijke Brill NV, Leiden, 2011