Photodissociation of methyl nitrate (and its fully deuterated isotopomer) isolated in an argon matrix at 17 K has been investigated by 266 nm laser photolysis. Characterization of the product species has been performed by transmission Fourier transform infrared spectroscopy. There are two parallel initial reaction channels, one of which is formation of formaldehyde (H2CO) and hydrogen nitryl (HNO2). The second channel is formation of formaldehyde and a previously unreported species, imine peroxide (HNOO). Quantum chemical calculations of the structure, vibrational frequencies, and infrared band intensities have been performed at the B3LYP/6-311++G**, MP2(full)/6-311++G**, and QCISD/6-311++** levels of theory. The calculated vibrational frequency patterns allowed us to confirm the detection and infrared spectral assignments of imine peroxide for the first time.