Polyaniline is a member of the class of electrically conducting polymers, having possible commercial applications such as coatings. Aqueous-based polyaniline coatings are preferred over organic solvent or concentrated strong acid based coatings because water is not a pollutant. The overall goal of this study was to further the development of water-based polyaniline coatings by an examination of the effect of acidic water (pH 1.18) and air on polyaniline. Knowledge of the effect of water on the structure, molecular weight, electrical conductivity, and diffusion of the dopant assisted in ascertaining whether polyaniline degraded with water exposure. Knowledge of how Fourier transform infrared (FTIR) spectra would be affected by dry air was important for determining if polyaniline was chemically stable with time. The results showed that the molecular weight, ultraviolet-visible and FTIR spectra, and carbon-to-nitrogen molar ratio in the polymer backbone all remained unchanged during acidic water immersion. The constant nature of these physical parameters showed a high degree of water stability. A chloride ion diffusion coefficient of 2.8 to 85 x 10(-9) cm(2)/h, the chloride concentration, and the electrical conductivity were also measured with the water immersion time. Aging polyaniline powders in a desiccator for 5 years showed no effect on the molecular structure, as indicated by the FTIR spectrum. (C) 2003 Wiley Periodicals, Inc.