Visible-light-induced oxygen atom transfer from NO2 to dimethylamine has been investigated in a low-temperature Ar matrix. The bimolecular reaction with the threshold wavelength of 585 nm occurred to form dimethylamine N-oxide ((CH3)(2)NHO) and NO. Unstable dimethylamine N-oxide was detected for the first time as an intermediate using an FTIR spectrometer. Further irradiation at a shorter wavelength (514.5 nm) caused the photodecomposition of dimethylamine N-oxide into N-methylmethyleneimine (CH2=NCH3) and water, which interacted with each other through the hydrogen bond. The reaction mechanism is discussed in comparison with that of a CH3NH2/NO2 system.