Nanosized spinel-type MgxFe3-xO4 complex oxide catalysts were prepared by coprecipitation and citrate gel methods and were characterized. The crystallization temperature of spinel particles prepared by citrate gel method is about 300 degreesC lower than that of those prepared by coprecipitation method. MgxFe3-xO4 catalysts prepared by citrate gel method have higher dispersity and smaller particle size, leading to higher activity for styrene oxidation with H2O2 as oxidant. The nonstoichiometric MgxFe3-xO4 catalysts (x < 1) are more active for styrene oxidation than the pure spinel MgFe2O4. The major reaction (similar to65-70 mol%) is oxidative C=C cleavage into benzaldehyde, and the minor reaction (10 mol%) is epoxidation and its further isomerization into phenylacetaldehyde. The effects of solvent, styrene:H2O2 molar ratio, reaction temperature and time on the conversion and product distribution were also studied. (C) 2003 Elsevier Science B.V. All rights reserved.