Aramid fibres are high-performance materials proposed in geotextiles for alkaline ground reinforcement. To study their durability in such environments, accelerated ageing has been carried out at pH 9 and at pH 11 for up to one and a half years. First, the lateral and longitudinal crystallite sizes have been determined before and after ageing under these conditions by Wide-Angle X-ray diffraction. Next, the tensile fracture surfaces have been observed after different ageing times by scanning electron microscopy. Finally, molecular weight changes have been evaluated by size exclusion chromatography. This study highlights the dependence of structural changes on the ageing conditions. A structural degradation scheme is proposed.