Infrared reflection absorption spectroscopy (IRAS) at low temperatures (ca. 40 K) has been used to study the interaction of NO with highly ordered MgO(100) thin films. Dimer formation has been observed in the multilayer regime at (<60 K) and in the monolayer regime at 60-70 K. Using (NO)-N-15, it is shown that dimeric NO, (NO)(2), is the key intermediate in the formation of N2O above 70 K.