Barium hexaaluminogallate was synthesized by the mixed-oxide method and the coprecipitation method. Barium hexaaluminate and barium hexagallate were found to form barium hexaaluminogallate, Ba(Al,Ga)(12)O-19, at 1400 degreesC. BaCo(Al,Ga)(11)O-19 of magnetoplumbite structure was formed from a mixture of metal oxides at 1200 degreesC for 6 h, Co was successfully introduced to barium hexaaluminogallate, although the synthesis of BaCoAl11O19 is quite difficult via solid-state reaction of oxide powders, Anisotropic BaCo(A1,Ga)(11)O-19 particles crystallized at 1100 degreesC for 2 h through the coprecipitation method using metal nitrates and ammonium carbonate. BaCo(Al,Ga)(11)O-19 supported on a cordierite honeycomb had the ability to reduce NOx with methane over 500 degreesC in the presence of excess oxygen.