After being subjected to preliminary purification, montmorillonite clay was leached with varying concentrations of sulfuric acid (0.5-2 mol L-1) at 90 degrees C for varying times (0.5-4 h). Acid leaching causes partial amorphization of the montmorillonite and increases its specific surface area by a factor of 3 (from 49.1 to 157 m(2) g(-1)). The bleaching efficiency for Mongolian mare's milk oil, as judged by light absorbance measurements at 400 nm, was increased from 9.5% for the unleached clay to 93.8% for clay leached with 2 mol L-1 H2SO4 for 4 h. (c) 2006 Society of Chemical Industry.