Manganese cerium oxides were synthesized by Pechini method and were used for investigating the performance of solid solution and the influence of water vapor presence on catalytic decomposition of formaldehyde and ozone at room temperature. Compared with the pristine manganese oxides and cerium dioxide, MnCeOx catalyst exhibited the best performance on HCHO oxidation and excellent ozone decomposition under dry air conditions although the CO2 yield is much lower than that of the solo cerium oxide. The as-prepared catalysts were characterized by XRD, XPS, N-2 adsorption-desorption, H-2-TPR and O-2-TPD techniques. These characterization results revealed that the MnCeOx catalyst formed a solid solution of manganese and cerium, and exhibited relatively abundance oxygen vacancies and the most surface lattice oxygen species, enhancing remarkable adsorption and redox properties. MnCeOx catalyst has similar to 100% HCHO conversion into CO2 in the catalytic activity test while the relative humidity is higher than 50% in this work, which could be suitable for indoor air purification. In situ DRIFTS results demonstrated that this excellent performance of HCHO complete oxidation is attributed to the continuously replenished surface hydroxyl groups generated from the interaction of water vapor and ozone on the surface of MnCeOx catalyst. This composite oxide is a promising catalyst for removing formaldehyde and ozone in the indoor environment.