Equilibrium data for the adsorption of methylene blue onto activated carbon was reported. The equilibrium data were splitted to different data sets to have an idea on the partial and complete isotherm. The equilibrium data were analyzed using Freundlich, Langmuir and Redlich-Peterson isotherm. The influence of partial isotherm and complete isotherm on the equilibrium parameters in isotherm expression were estimated. Equilibrium data covering the complete isotherm is the best way to obtain the parameters in isotherm expressions. Present investigation showed that for successful batch sorber design, equilibrium data with partial isotherm is not sufficient, instead equilibrium data that covers complete isotherm is required. Redlich-Peterson and Langmuir isotherm well represent the adsorption of methylene blue onto activated carbon. Redlich-Peterson isotherm is a special case of Langmuir when the constant 'g' equals unity. (c) 2005 Elsevier B.V. All rights reserved.