The removal of an acid dye from water onto raw montmorillonite and HCl-activated montmorillonites in fixed beds was investigated. Experiments were carried out as a function of liquid flow rate, feed dye concentration, and bed height. The adsorption capacity of acid dye onto raw clay could be largely improved when the clay was activated by HCl. The increase of adsorption capacity was due to the Al3+ or Fe2+ ions of montmorillonite replaced by W ion after HCl acidification, which was indicated by surface area (BET) and X-ray diffraction analysis. The mass transfer models which involve two parameters of maximum adsorption capacity (NO) and kinetic constant (k(AB)) from Adams-Borhart model and two parameters of kinetic coefficient of the external mass transfer (P.) and maximum adsorption capacity (N-0) from Wolboraska model were proposed. Those models could satisfactorily describe the measured breakthrough curves of acid dye in fixed beds (standard deviation < 5%). The effect of the type of clay (raw, HCl-activated) on the values of N-0, k(AB), and beta(a) was discussed, and the application potential of acid-activated clay for adsorption removal of acid dye from water was also demonstrated.