Clinoptilolite-containing rocks were used to study the possible uptake of arsenite (H 3 AsO 3 ) and arsenate (H 2 AsO 4 - ) species from aqueous solution. Two samples also exhibited mordenite and erionite zeolitic phases. Batch and isotherm studies at pH 4 in the concentration range 0.1<C< 4 mgl - 1 were performed. The experimental data were used to model Freundlich, Langmuir and Langmuir-Freundlich adsorption isotherms. Solutions were analyzed by means of ion chromatography coupled with inductively coupled plasma mass spectrometric detection (IC-ICP-MS). All zeolitic samples investigated removed more H 2 AsO 4 - than H 3 AsO 3 at equivalent arsenic concentrations. Although the saturation capacity of the tuffs studied was inversely related to the silicium dioxide content and directly to the iron content in the acid-washed zeolite, the addition of iron did not significantly improve the removal efficiency. Up to 98% of H 3 AsO 3 were removed from a 500 μg As l - 1 solution by three of the samples studied after a contact time of 70 days.