The diffusion behavior of ions in polyaniline has been studied by in situ electrochemical W-visible spectroscopy. The fibrous polyaniline used was prepared by electropolymerization of aniline in an acidic aqueous solution at various current densities. The radius of the polyaniline fibril was dependent on the current density. Theoretical time transients of UV-visible absorption during doping and undoping of ions into/from polyaniline were calculated from a cylindrical diffusion model. From this, the estimated diffusion coefficient was found to increase with increasing radius of the polyaniline fibril, suggesting the diffusion coefficient is related to the physical property of polyaniline and the electropolymerization current density. A similar dependence has been observed when using a nonaqueous electrolyte. However, the diffusion coefficients obtained in acid solutions were much larger than those measured in the nonaqueous electrolyte. It is, therefore, believed that the diffusing species in polyaniline immersed in an acidic aqueous electrolyte differs from those of polyaniline immersed in nonaqueous electrolytes.