Altervalent cation-doped neodymium sesquioxide systems such as Ni/Nd2O3, Mn/Nd2O3, and Zr/Nd2O3 with various doping mole fractions were prepared and the cation-doping effect on the catalytic activity of neodymium oxide in the oxidative coupling of methane was investigated. The catalytic reaction was carried out in a flow reactor system using on-line gas chromatography. The reaction conditions were 550-800 degrees C, feed mole ratio of CH4/O-2/He=6/1/5, total feed flow rate=30.0 cm(3) min(-1), and 1 atm of pressure. The present catalysts were effective for the oxidative coupling of methane, the selectivity to higher hydrocarbons was increased by doping the cations into Nd2O3. Among the catalysts tested, 5 mol% Zr-doped Nd2O3 showed the best C-n-selectivity (n greater than or equal to 2) of 74.1% with a yield of 13.0% at 750 degrees C. Pure Nd2O3, Ni-doped Nd2O3, and Mn-doped Nd2O3 catalysts showed no C-3-hydrocarbon product selectivity, while Zr-doped Nd2O3 catalysts showed C-3-hydrocarbon product selectivity ranging from 6% to 12%. When the oxygen-pretreated 5 mol% Ni-doped Nd2O3 catalyst was exposed to CH4 at 400 degrees C, a detectable amount of H-2 was produced, indicating that CH4 can be selectively activated by oxygen species on the surface. The formation of interstitial oxygen ions through the reaction of oxygen with the oxide is a controlling factor for the catalytic activity of the oxide in the oxidative coupling of methane. The oxygen vacancy formed by doping divalent cation into Nd2O3 exerts influence on the formation of active oxygen ion. Zr4+-doping can also increase the concentration of active oxygen ion in the oxide. Defects and active sites in the catalyst are discussed on the basis of solid-state chemistry.