The aim of this study was to compare the reactivity of three lanthanum-based phases in presence of methane CH4 or carbon monoxide CO gases. The hydroxycarbonate LaOHCO3 was synthesized from a new specific precipitation route. From this hydroxycarbonate phase, the dioxycarbonate La2O2CO3 and the oxide La2O3 were prepared by thermal decomposition process. The polycrystalline samples were characterized by X-ray diffraction, scanning (SEM) and transmission (TEM) electron microscopy. Statistical analyses of grain sizes and morphologies were carried out from TEM analyses. The three samples were then subjected to interactions with air-CH4 and air-CO flows, at various temperatures. The oxidation of CH4 and CO into CO2 interacting with the samples was studied using Fourier Transform infrared spectroscopy. Finally, the reactivity of samples is discussed.