The ability of chitosan, derived from deacetylated crab shell chitin, to remove acid dyes from effluent solution by adsorption has been studied. Equilibrium isotherms for the adsorption of five acid dyes, Acid Green 25 (AG25), Acid Orange 10 (AO10), Acid Orange 12 (AO12), Acid Red 18 (AR18) and Acid Red 73 (AR73), on chitosan were measured experimentally. Results were analysed by the Langmuir, Freundlich and Redlich-Peterson (R-P) equation using a linearised correlation coefficient. The Langmuir isotherm gave the best correlation for the adsorption of AG25, AR18, AR73 and AO12 while,the composite Freundlich isotherm gave excellent correlation for AO10. The equilibrium data were then analysed using five different non-linear error analysis methods and two significant findings were obtained. Firstly, the order of isotherm selection was changed showing that the R-P equation had the lowest error values for all five acid dye-chitosan isotherms. Secondly, the values of the individual isotherm constants changed with the error methodology selected. (C) 2003 Elsevier Ltd. All rights reserved.