We have conducted differential water vapour sorption experiments on KevlarTM 49 at 30°C over a series of water vapour pressures from 0 to 90% of saturation, and on the as-polymerised form of the material at 30°C, 45°C and 60°C over a series of water vapour pressures of 0–60%, 0–25% and 0–15%, respectively. The equilibrium isotherms obtained for both samples show a distinct hysteresis-type behaviour. For KevlarTM 49, the hysteresis loop can be divided into two regions, namely above 30%, which is indicative of the presence of microvoids, and below 30%, which suggests inclusion of water into the intimate structure of the surface layer of the polymer crystallites, in a process known as intercalation. [Copyright &y& Elsevier]