The effects of the substitution of transition-metal ions and/or reductant gases on the catalytic properties of barium hexaa-luminogallate were investigated. Transition-metal-substituted hexaaluminogallates (BaM(AI,Ga)(11)O-19, M = transition metal, AI/Ga = 9/3) were synthesized from aqueous metal nitrates and ammonium carbonate by the coprecipitation followed by crystallization at 1100degreesC. The direct NO, reduction was observed over BaM(AI,Ga)110,9 to be around 10%. The NO, removal activity of BaM(AI,Ga)(11)O-19 powders was improved by addition of C3H6 as a reductant gas. Co-, Ni- and Cu-substituted BaM(AI,Ga)(11)O-19 catalysts exhibited about 40% NO, reduction with C3H6, in excess oxygen at a high space velocity of 10 000 h(-1). The NO, reduction on Mn- and Fe-substituted BaM(AI,Ga)(11)O-19 catalysts was less than 10% even in the presence of C3H6,. The temperature of the effective NO, reduction on BaM(AI,Ga)(11)O-19 catalysts could be adjusted from 350degrees to 500degreesC by the selection of the transitionmetal substitution in the catalysts. The catalysts hold high activities for NO, reduction even at 500degreesC in water vapor produced in the combustion system of reductant gases.