XPS and IR spectroscopies were used to investigate the surface intermediates of dimethyl ether (DME) oxidation to formaldehyde over MoOx/Al2O3 catalyst. The reaction performances were tested by employing three typical reaction conditions, depending on the O-2/DME ratio and the reaction temperature. When there was sufficient oxygen present in the reaction media, a terminal or bridged CH3O* species formed by DME dissociation was highly active and rapidly reacted with lattice oxygen to produce formaldehyde, leading to higher selectivity of HCHO. When oxygen was consumed completely or only DME was present in the reaction media, CH3O species bonded to more than two Mo atoms (mu-OCH3) and CHx (x=1-3) species attached to the Mo atoms were observed and the relative ratio of (mu-OCH3) /Mo-CHx was significantly dependent on the reduction degree of MoOx domains. The (mu-CH3O) species was related to the formation of CH3OH or COx, and the Mo-CHx species led to the formation of CH4.