A series of non-stoichiometric magnesium-aluminate solid solution spinels, used as sulfur-transfer catalysts in the fluid catalytic cracking units for SOx emissions abatement, were prepared by using coprecipitation method and characterized with BET, TGA, AES, XRD and in situ IR techniques. It was found that both the crystalline structures and De-SO., activities of magnesium-aluminate spinels are very sensitive to catalyst compositions. Several phase domains were produced by changing the mole ratio of alumina to magnesia in the preparation process. Owing to substitution of magnesium or aluminum ions in the spinel structure, that leads to a polarization of Al3+ or Mg2+ ions to adjacent oxygen lattices, the lattice cell parameter of spinel structure regularly varied with chemical compositions, producing a contraction or expansion effect in the lattice cell, that strongly affects De-SOx activity. TG analysis showed that during SO2 oxidative adsorption, most of sulfur species were captured on the surface and some sulfur species were stored in bulk of the solids. The IR and AES results confirmed that both surface and bulk-like sulfates with different H-2-reducibilities were formed on the catalysts, which is in good agreement with results of SO2 monolayer adsorption measurement. Ten-cycle tests of SO2 oxidative adsorption and reductive decomposition of the formed sulfate showed that the sample with X-Al = 0.33 is the optimum catalyst, its SO2 capturing capacity reached 124.4 mg/g. (C) 2002 Elsevier Science B.V. All rights reserved.