Manganese oxide nanocrystals are combined with aluminum oxide nanocrystals to improve their crystallinity via calcination without a significant increase of crystal size. A nanocomposite, consisting of two metal oxides, can be synthesized by the reaction between permanganate anions and aluminum oxyhydroxide keggin cations. The as-prepared manganese oxide-aluminum oxide nanocomposite is X-ray amorphous whereas heat-treatment gives rise to the crystallization of an alpha-MnO2 phase at 600 degrees C and Mn3O4/Mn2O3 and. gamma-Al2O3 phases at 800 degrees C. Electron microscopy and N-2 adsorption-desorption-isotherm analysis clearly demonstrate that the as-prepared nanocomposite is composed of a porous assembly of monodisperse primary particles with a size of similar to 20 nm and a surface area of >410 m(2) g(-1). Of particular interest is that the small particle size of the as-prepared nanocomposite is well-maintained up to 600 degrees C, a result of the prevention of the growth of manganate grains through nanoscale mixing with alumina grains. The calcined nanocomposite shows very-high catalytic activity for the oxidation of cyclohexene with an extremely high conversion efficiency of >95% within 15 min. The present results show that the improvement of the crystallinity without significant crystal growth is very crucial for optimizing the catalytic activity of manganese oxide nanocrystals.