The ion exchange equilibrium of Pb(II) on clinoptilolite modified with NH4Cl and NaCl can be represented by two types of isotherms. The first one is the ion exchange isotherm based upon the constant of thermodynamic equilibrium for the ion exchange reaction; however, the fitting procedure for this isotherm can be very tedious due to all the calculations involved and additional thermodynamic data. The second one is the Langmuir isotherm. The use of the Langmuir isotherm to represent ion exchange equilibrium has increased in recent last years since it adequately fits the equilibrium data and, furthermore, its calculation is much simpler. A comparison between the two isotherms showed that they fitted the experimental data reasonably well, but the Langmuir isotherm is much simpler and easier to use. (c) 2006 Elsevier Inc. All rights reserved.