The reaction of ground-state NH with NO has been investigated in solid xenon matrix. NH radicals are formed in good yield in Xe matrix after UV photolysis of HN3 and are reacted with NO in the matrix by annealing the solid to 40 K. A photolabile product is formed and identified as trans-HNNO by infrared spectroscopy, including isotope labeling and comparison with published frequencies from quantum chemical calculations. A minor product with similar formation and photodestruction behavior is assigned to the cis-HNNO isomer. Cis-trans interconversion was also observed. The reaction to form HNNO is essentially activationless. Upon photolysis at wavelengths up to 600 nm, HNNO is destroyed and converted to N2O. This is the first detection of this intermediate, which plays a role in ammonia combustion and NOx removal schemes.