Magnesium aluminate (MgAl2O4) platelets were first synthesized by hydrothermal treatment of gamma-AlO(OH) in a magnesium nitrate aqueous solution at 400 degrees C. The platelets are 100-200 nm in width and 25 nm in thickness. The influence of temperature, the anions of the magnesium salt, the amount of magnesium salt, and precursor pH on the formation of such structure was investigated. The experimental results indicated that the anions of the magnesium salt and precursor pH could have a significant influence on the growth of MgAl2O4. MgAl2O4 formation followed the in situ transformation mechanism, which was the reason why the MgAl2O4 crystallites had a platelet morphology.