The behavior of chitosan nanoparticles as an adsorbent to remove Acid Green 27 (AG27), an acid dye, from an aqueous solution has been investigated with nanochitosan (particle size = 180 nm; degree of deacetylation = 74%). The dye concentration at equilibrium (Q(e), mg/g) was calculated using the weight of the nanoparticles in the mixed solution (Q(es)) and the weight of chitosan in the nanoparticles (Q(ep)). The experimental isotherm data were analyzed using the Langmuir equation for each chitosan sample; the Langmuir monolayer adsorption capacity (Q(o)) was calculated with Q(es) and Q(ep) and the results were 1051.8 mg/g and 2103.6 mg/g, respectively, which were significantly higher than that of the micron-sized chitosan. (c) 2006 Elsevier Ltd. All rights reserved.