Chitosan is a biopolymer found in the body of the shellfish, crustaceans and some fungi. The free amino groups of this biopolymer show different properties such as anti-microbial, anti-acid and chelating of metal ions. Chitosan has been used as an adsorbent for the removal of the metal ions from industrial effluents. In this paper, the sorption of mercuric ions from solution onto chitosan particles with sizes of 0.177, 0.5 and 1.19 mm at three initial pH 3.00, 4.50, 6.00 +/- 0.1 has been investigated. The experimental data were analyzed using the Langmuir, Freundlich and Redlich-Peterson equations. Error function and correlation coefficients were determined for each isotherm analysis. The results showed that increasing of pH increased the extent of Hg(II) ions uptake and decreasing of adsorbent particle sizes increased the amount of mercury removals. The study of equilibrium isotherm shows that the best model for analysis of experimental data is Langmuir model with correlation coefficient higher than 0.982. Maximum Hg(II) ions uptake with chitosan in our experiments (from Langmuir equation) is 1127.1 (mg Hg2+/g chitosan). (C) 2007 Elsevier B.V. All rights reserved.