A polyaniline (PANT) was synthesized by the oxidative polymerization using ammonium persulfate as an oxidizing agent. The PANT was then stirred with excess fuming sulfuric acid at room temperature for 6 h to obtain water soluble sulfonated polyaniline (SPANI). The degree of sulfonation was found to be 93-94% from the Fourier transform infrared (FTIR) and elemental analysis. The solubility of the SPANI in water was 1.25 g/L at room temperature and appeared as a green color solution. Conductivity of the PANT was decreased after sulfonation. A proliferation of hydrophilic nature of the PANT after sulfonation was observed from the water contact angle measurement. From the UV analysis, it was revealed that the energies required for the pi-pi* and bipolaron/polaron transitions are less and the intensity of these transitions are lower in SPANI compared to those of PANT. A detailed study on the crystal structures of PANT and SPANI were accomplished from the powder X-ray diffraction analysis. The SPANI exhibited a more ordered structure having a higher degree of crystallinity and crystallite sizes with an increased unit cell volume compared to the PANT. After sulfonation the morphology of PANT was transformed from a rod-like shape to a flat-plate shape. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2025-2035, 2010