Abstract Acid activation with HCl of two bentonites from the Aegean Islands of Milos and Chios, Greece, consisting of Chambers and Tatatilla-type and Otay-type montmorillonite, respectively, resulted in a 4 to 5-fold increase of the surface area of the raw materials. The activated materials have been rendered suitable for decolorization (bleaching) of rapeseed oil through removal of β-carotene. The optimum bleaching capacity is not associated with maximum surface area. Activation is characterized by destruction of the original smectite structure, removal of octahedral cations, uptake of OH − and formation of au amorphous Si-rich phase. Mg is the most readily removed element affecting the tendency for activation. The Otay-type montmorillonites are activated. more easily. Optimum conditions for activation are obtained using a variety of combinations of acid strength and residence time. The combination which is likely to be preferred on an industrial scale, is the least energy consuming. Therefore shorter treatments with more dilute acid are preferable.