We have made Pt/MgO catalysts by cogelling magnesium ethoxide and hexachloroplatinic acid. The reduction capability of the hydrolysis reaction brings about the formation of metallic platinum in air, even at 70 degrees C. In the course of the magnesium alkoxide gelation, platinum catalyzes the synthesis of brucite crystallite elongated along the (110) direction. Platinum also promotes the formation of small MgO crystallites, which are highly magnesium deficient. Platinum atoms, however, do not incorporate into the periclase lattice.