We have conducted differential water vapour sorption experiments on Kevlar(TM) 49 at 30degreesC over a series of water vapour pressures from 0 to 90% of saturation, and on the as-polymerised form of the material at 30degreesC, 45degreesC and 60degreesC 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 Kevlar(TM) 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. (C) 2004 Elsevier Ltd. All rights reserved.