The electron transfer at the interface between an aqueous solution (W) containing flavin mononucleotide (FMN) and an organic solution (Org) containing decamethylferrocene in the absence and presence of oxygen was studied by polarography. The reduction of FMN in the absence of oxygen at the W/Org interface differed from that observed by voltammetry at metal or carbon electrodes. The reduction of FMN proceeds as a one-electron process at the W/Org interface, whereas a two-electron process is observed at the metal or carbon electrodes. In the presence of oxygen, the reduction of oxygen by chemical reaction with the reduced FMN was observed at the W/Org interface. Mechanisms of reduction processes of FMN and oxygen at the W/Org interface are discussed.