Supported Co3O4 catalysts were prepared via the calcination of Co-containing hydrogarnet, (Ca3-xCox)Al-2(SiO4)(3-y)(OH)(4y), at 400degreesC. Such precursors with various extents of substitution were synthesized from the hydrothermal reaction of a stoichiometric mixture of calcium oxide, amorphous silica, alumina sol, and cobalt hydroxide at 200degreesC. It was found that the catalyst consisted of Co3O4, CaO, and mayenite and exhibited a high activity for the combustion of propylene, benzene, and toluene at temperatures below 300degreesC.