The interactions between heavy metals (vanadium and nickel) and kaolin had been investigated by X-ray diffraction and MAT test. In the process of phase transformation of kaolin containing 1-5% vanadium, vanadium-mullite phase emerged surprisingly at 660-700degreesC. The unit cell size of the mullite was found to increase with increment of vanadium content. As compared with hydro-kaolin, acid-modified kaolin (AMK) and caustic-modified kaolin (CMK) reacted with vanadium to form much more mullite; CMK could even be reacted with nickel to form a rather stable NiAl10O16 compound, and this compound protected zeolite in fluid catalytic cracking catalyst effectively. The catalyst containing AMK had greater MAT activity retention than that of the contrastive catalyst with hydro-kaolin in the identical level of heavy metal contamination. (C) 2003 Elsevier B.V. All rights reserved.