MnO (x) -CeO2 catalysts were synthesized using the citric acid complex method with natural or complexation aging in order to investigate the oxidation of formaldehyde at ambient temperature. Compared with different preparation method, such as natural aging (NA), complexation aging (CA) and co-precipitation process (CP), the sample with CA exhibited high activity maybe owing to high surface area, smaller particle diameter, water absorbability, as well as stronger interaction with MnO (x) and CeO2. The catalysts exhibited the highest activity when using CA with the Mn/(Mn + Ce) molar ratio (f(m)) of 0.8. The highest activity of f(m) = 0.8(CA) was attributed to the micropore structure, amorphous state for the MnO (x) -CeO2 solid solution, low reduction temperature and excellent oxygen adsorption. Among these factors, the chemically adsorbed oxygen from the vacancies (O-) or weak surface lattice oxygen (O2-) was the most important factors for catalytic oxidation of formaldehyde at ambient temperature. [GRAPHICS] .