Measurements are reported for the sorption equilibrium and transport of water into commercially produced polyaniline solid fibers from a 50% relative humidity air stream at ambient pressure of similar to 0.1 MPa and 300 K. The data have been collected using a single solid fiber morphology, PANION, but with different acid dopants used to change the polyaniline from its emeraldine base (insulating), PANI-EB, to the emeraldine salt form (conducting), PANI-ES. The sorption process was well described by unsteady Fickian diffusion into an infinite cylinder. The rates and equilibrium capacities depended on the acid dopant utilized, but did not vary monotonically with the anion size or the acid strength. The measured adsorption/desorption water capacities varied between 20 and 75 (mg of H2O)/(g of dry polymer), depending on the anion dopant. The BF4- and H2PO4- doped fibers had the highest capacities. When the mass loading was recalculated on a (1120 molecule/polymer repeat unit) basis, most doped fibers had a capacity of similar to 0.5 H2O per PANI-ES repeat unit. The exceptions were the BF4- and H2PO4- doped fibers, for which the value was similar to 1. The apparent diffusion coefficients varied between 0.05 and 0.6 x 10(-12) m(2) Is and were usually larger during adsorption than desorption. The water capacity of the polyaniline solid fibers is at the upper end of what is usually observed for glassy polymers and provides the possibility for exploiting electronic conductivity, good mechanical strength properties, and desiccant qualities for advanced humidity control and sensing applications.