The precursor particles for gamma-Ga2O3-Al2O3 solid solutions were prepared by the coprecipitation method from aqueous solutions of Ga(NO3)(3) and Al(NO3)(3) with (NH4)(2)CO3 as a precipitant. The gamma-Ga2O3-Al2O3 solid solutions were obtained by calcination of the precursor at 700 A degrees C. In this paper, the performance of the catalysts treated with NH3 was investigated for the selective catalytic reduction (SCR) of NO with methane as a reducing agent, and it was found that gamma-Ga2O3-Al2O3 catalysts treated with NH3 and subsequently annealed in air showed higher activities than the gamma-Ga2O3-Al2O3 catalysts without NH3 treatment. NH3 treatment of the catalyst caused partial rearrangement of Ga3+ and Al3+ ions and increased the population of tetrahedral Ga3+ ions in the defective spinel structure.