The influence of moisture on the activity and stability of Mn0.75Co2.25O4 catalyst for HCHO and CO oxidation was compared and investigated. Totally opposite effects of relative humidity (RH) on the reactions were observed: with increase of RH (0-90%), the activity for HCHO oxidation was enhanced, whereas the activity for CO oxidation was greatly decreased. However, in both cases the catalyst lifetime was prolonged in the presence of moisture (RH = 50%). Such modifications were directly evidenced by correlating in-situ DRIFTS data, TPO results and MS-IRAS studies, which clarified the effects of water on the activity and stability of Mn0.75Co2.25O4 catalyst for HCHO and CO oxidation. (C) 2014 Elsevier B.V. All rights reserved.