The adsorption of nitrogen on silica-supported iridium has been studied by infra-red spectroscopy. A band at 2185 cm–1 was observed on “bare” iridium and is assigned to a species of chemisorbed nitrogen with the electronic structure [graphic omitted]. Heats of adsorption for this species (2-4 kcal/mole) have been estimated from the variation of band intensity with temperature at constant pressure.