The effect of penetrants, viz., moisture, aqueous alkali halide solutions and some common organic solvents on the aromatic polyamides Kevlar and Nomex has been analysed. The saturation moisture content in Kevlar fibres is dependent primarily on sample characteristics. The dynamic process of moisture uptake is, however, controlled by the relative humidity of the ambient atmosphere. In case of organic solvents, the extent of retention by Kevlar depends on the molar volume as well as the presence of specific functional groups in the former. In contrast with Kevlar, Nomex sheets are found to be impervious to most of the penetrants.