Atomistic classical molecular dynamics simulations have been used to investigate the water absorption behavior of polyaniline emeraldine base. Results derived from simulations, which were performed considering polymeric systems with different concentrations of water (3%, 10%, 15%, 20%, 50%, and 100% w/w), have been compared with experimental evidences obtained for both powder and films. Calculation of the Hilde-brand solubility parameters explains not only the insolubility of polyaniline in water but also the requirements, in terms of intermolecular interactions, needed by solvents to be compatibles with this polymer. The maximum content of absorbed water predicted for hygroscopic polyaniline is 15% w/w. The effects of the absorbed water in both the organization and conformation of the polymer chains have been examined at the microscopic level by analyzing the microstructures derived from simulations, while the role of the moisture in the morphology of powder and film samples has been investigated using atomic force microscopy. The overall of the results provides a complete and understandable view of the polyaniline water interactions, and explains the influence of the water molecules in the structural properties of the polymer. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1322-1331, 2011