A progressive decrease in cation exchange capcity (CEC) values was observed by treating Ca-montmorillonite with sulphuric acid solutions and this can be understood in terms of the layered structure of the clay. Elemental analysis showed that moderate activation occurred and only 25-30% of the octahedral cations were removed. At the same time the total surface area and the clay acidity increase. X-ray and FTIR data confirmed that acid activation affects both the octahedral and the tetrahedral sheets. The efficiency of acid-activated montmorillonite for the bleaching of cottonseed oil was investigated. The differences in bleaching efficiency appeared to be due to differences in the physical and chemical properties of the bleaching media. The oil acid value was not affected by the bleaching procedure but a slight shift in the absorption maximum of the bleached cottonseed oil was observed. Medium activation of the clay (treatment of Ca-montmorillonite with 4 N H 2 SO 4 ) was the most effective in bleaching the cottonseed oil, resulting in the best colour index and the lowest peroxide value. A linear dependence of the bleaching efficiency on the clay surface area and acidity was observed. The role of the increased Bronsted acidity is also discussed.