The reaction between molecular oxygen and the imidogen radical NH in their ground electronic states has been investigated in solid cryogenic rare gas matrices. A photolabile product forms upon annealing a xenon matrix containing oxygen and trapped NH radicals produced by in situ UV photodissociation of hydrazoic acid. The product is identified as imine peroxide, HNOO, by its infrared spectrum and its photoisomerization to nitrous acid, with support from the calculated vibrational spectrum and O-18 isotope shifts. The reaction is essentially activationless. Some aspects of the potential energy surface for this species are delineated by the conditions under which HNOO is formed and destroyed. This is the first detection of this compound, a 1,3-dipole isoelectronic with ozone and peroxymethylene.