Abstract The adsorption of barium by Ca-exchanged clinoptilolite and montmorillonite is presented. The kinetics of adsorption of Ba 2+ were evaluated contacting 1 g portion of each adsorber with 100 mL 0.1N BaCl 2 for 200 h. Adsorption by Ca-clinoptilolite is defined by second-order kinetics of rate constant K v 8.232 × 10 −2 g mg −1 h −1 and maximum removal of 71.885 mg g −1. It is a two-stage process initiated by a rapid uptake of Ba 2+ followed by more moderate kinetics. The adsorption isotherms were determined contacting 0.2 g of each adsorber with 10 mL 0.1–0.005N BaCl 2 + CaCl 2 solution, Ba 2+/Ca 2+ ratio 1, for periods of 7 days for the tuff and 2 days for the clay. The equilibrium adsorption is described by the Langmuir model, of equilibrium constant K 0.0151 L mg −1 and maximum adsorption of 15.29 mg g −1. The adsorption of Ba 2+ by Ca-exchanged montmorillonite also follows a second-order reaction of rate constant K v 3.179 × 10 −2 g mg −1 h −1, and calculated separation of 36.74 mg g −1; the Langmuir isotherm is defined by the constant K 0.034 L mg −1 and maximum adsorption of 15.29 mg g −1. X-ray diffraction shows that the exchange of Ba 2+ modifies the d 0 0 1 of Ca-montmorillonite from 15.4 to 12.4 Å.