Self-doped sulfonic acid ring-substituted polyaniline (SPAN) reported so far is only slightly soluble in water. This work reports the synthesis route of SPAN in water with a solid content at least 50% by weight, which is stable (without precipitation) for more than 1 year and can be cast into freestanding films. The protons on the bound sulfonic acid groups can dope the imine nitrogens both in its aqueous form and as a solid film as evidenced by the presence of polaron absorption bands in its UV-vis spectra and of free spins from the ESR measurement. Titration analysis and spectroscopic data of the polymer in aqueous solution and as solid film, respectively, show that, surprisingly, the fractions of -SO3H groups that protonate (dope) the imine nitrogens to give semiquinone radical cations are equal in both cases, about 0.60 (equivalent to the doping level, 0.30). However, the free spins in the aqueous solution are more localized than those in the solid film. The thermostability of SPAN is also investigated. The conductivity of SPAN film (at 25 degrees C, 0.01 S/cm) increases with temperature from -10(0.002 S/cm) to 190 degrees C (0.09 S/cm) and decreases with temperature from 190 to 225 degrees C (0.06 S/cm) due to thermal undoping accompanying a loss of some polarons as confirmed by the UV-vis, LR, and TGA analyses. After the heating scan, the conductivity at room temperature drops by a factor of 10(3).