The thermal reaction of (4-nitrophenyl)nitrene with oxygen in low-temperature matrices produced a new molecular species, which was identified as 4-nitrophenyinitroso oxide on the basis of its IR spectrum in combination with isotopic labeling with O-18(2) and vibration analyses by the DFT method. This is the first report for the infrared detection of an aryl nitroso oxide. We could monitor also the wavelength-dependent N-O and O-O bond cleavage of the nitroso oxide.