Polyaniline (PANI) forms thermoreversible gels in four different sulfonic acids, e.g., dinonylnaphthalene sulfonic acid (DNNSA), dinonylnapthalene disulfonic acid (DNNDSA), +/-camphor-10-sulfonic acid (CSA), and dodecyl sulfonic acid (DSA), when made from the formic acid medium. The gelation behavior of 15% PANI-sulfonic acid (weight to weight) gels are reported here. The morphology, studied from the scanning electron microscopy (SEM) and transmission electron microscopy (TEM), indicates the presence of fibrillar network structure in all of the systems. The thermal study of the gels indicates reversible first-order phase transition during both cooling and heating processes in a differential scanning calorimeter (DSC-7). The WAXS patterns of the gels are different from that of the PANI (EB) from and from each other. Sulfonic acid-subtracted FT-IR spectra of the gels indicate presence of new peaks due to gelation. From the WAXS, electron diffraction, and thermal investigations of the gels, it has been inferred that crystallization is the cause of gelation. The conductance of the gels have an order of 10(-2) to 10(-1) S/cm at 27 degrees C. A lamellar model for the gel structure, consisting of PANI layer and sulfonic acid layers, is used to explain the gel, and the thermoreversibility is believed to be due to the crystallization of the extended tails of the sulfonic acids under the doped condition.