Natural zeolite (NZ) obtained from the Manisa-Demirci region of Turkey was modified by NaOH aqueous solutions (0.5-2.0 mol/dm(3)) and the adsorption capacity of natural and modified forms was determined for manganese removal. The characterisations of the zeolites were carried out by XRD, N-2 sorption, FTIR, NH3-TPD and SEM-EDS. Treatment of the NZ with NaOH leads to a significant decrease of its silica content by desilication and an increase of its Na content by formation of hydroxysodalite. It was seen from the NH3-TPD results that NaOH treatment alters the acidity of zeolite. While the number of weak and medium Bronsted/Lewis acid sites decreases, the number of strong acid sites increases through the intraduction of Na+ and the removal of silica. Treatment with NaOH has the effect of multiply by two the manganese adsorption capacity of the natural zeolite. The maximum manganese adsorption capacity was achieved with the zeolite treated with 1.5 M of NaOH. More than this concentration of NaOH leads not only to a decrease in adsorption capacity of natural zeolite for manganese, but also to a significant deformation of the zeolite structure. The Langmuir isotherm model fits well with the results obtained from manganese adsorption on natural zeolite, while the Freundlich isotherm model fits well with the results obtained on NaOH modified zeolites. (C) 2015 Elsevier B.V. All rights reserved.