The effects of promoters on the performance of a VOx/SiO2 catalyst were investigated with the goal of increasing its catalytic activity for the oxidation of methane to formaldehyde. Fourteen types of metal oxides (MOx, M = Ga, In, Y, Al, Bi, Zr, La, Zn, W, Mg, Mo, Ti, Ce, and Ca) were tested as promoters. Among them, Ga2O3, In2O3, Y2O3, and Al2O3 substantially increased the methane conversion rate and the formaldehyde yield, whereas MgO, MoOx, TiO2, CeO2, and CaO were less effective as promoters. The catalytic activity also depended on the amount of the promoter and the method by which it was loaded onto the support. The highest activity was obtained over VOx/Ga2O3/SiO2 (V loading, 2-3 wt%; Ga loading, 0.1-0.5 wt%) prepared by a sequential impregnation method (Ga loaded first and then V): under the standard reaction conditions (reaction temperature, 590 degrees C; gas hourly space velocity, 60,000 L kg(-1) h(-1) CH4/O-2 ratio, 2), the formaldehyde yield over this catalyst was 2.1 times that over bare VOx/SiO2. Reactions over the VOx/Ga2O3/SiO2 catalyst and the bare VOx/SiO2 catalyst showed different kinetics, indicating that Ga2O3 altered the properties of the active VOx sites. Catalyst characterization by means of X-ray diffraction and temperature-programmed reduction under H-2 suggested that the high activity of VOx/Ga2O3/SiO2 was due to the promoter's having increased the reducibility of the active VOx species without decreasing their dispersion.