Polyaniline (PANI) nanoparticles in the bulk solution were synthesized by the oxidative polymerization of aniline and ammonium peroxodisulfate as an oxidant in aqueous medium without the aid of any acid. The scanning electron microscopy showed PANI nanoparticles with average diameters in the range of 20 +/- 5 nm. Characterization of the prepared PANI nanoparticles by UV-visible spectroscopy, X-ray diffraction and Fourier-transform infra-red spectroscopy have been carried out. In addition, the thermogravimetric analysis and morphology using scanning electronic microscope are analyzed. The adsorption of a cationic dye, methylene blue (MB) using the PANI nanoparticles and conventional PANI was studied and discussed. The kinetic models: pseudo first order, pseudo-second order and the intraparticle diffusion models were applied to the experimental data. It was observed that pseudo-second order kinetic model described the adsorption process better than any other kinetic models. Results obtained indicate that PANI NPs base could be employed as an efficient adsorbent much more than the conventional PANI powder for dye removal. (C) 2012 Elsevier B.V. All rights reserved.