NO reduction with C3H6 in the presence of O-2 was studied over a mechanical mixture of Ni-Ga mixed oxide with 10 wt.% Mn2O3. Three Ni-Ga mixed oxides were prepared from co-precipitation methods by using urea or by using aqueous ammonia at the final pH of 8.5 and 9.5. All the Ni-Ga mixed oxides converted to NiGa2O4 spinel. By mixing them with Mn2O3, NiGa2O4 exhibited a NO reduction activity quite different from one spinel to another: the spinel prepared using urea exhibited a higher NO conversion than the spinel prepared using ammonia. In addition, the NiGa2O4 spinel prepared by using urea had larger pores, a smaller particle size and better Ni dispersion. These factors are probably responsible for the higher activity of the catalyst prepared by using urea. Another advantage of preparation using urea was that the Ga/Ni atomic ratio in the sample could be readily controlled. Using aqueous ammonia led to a serious Ni loss because of the formation of soluble Ni(NH3)(4)(2+) complex ions and as a result, the final Ga/Ni atomic ratio in the product was larger than the starting Ga/Ni atomic ratio. The synergetic effect of NO oxidation over Mn2O3 particles and the subsequent reduction of NO2 with C3H6 Over a neighboring NiGa2O4 spinel played an important role in the catalysis of the present mixture catalyst.