Laboratoire de Chimie Organique Physique -C.P. 160/08, Universite Libre de Bruxelles -50, av. F.D. Roosevelt, 1050 Bruxelles (Belgium) Dedicated to Professor Jacqiies Nasielski (ULB) oti the occasiori of his retiretiterit ABSTRACTThe luminescence spectra of tris(pheny1pyridine)iridium embedded in Arnberlite (polystyrene adsorbent) have been measured under various 0dN2 mixtures (total pressure = 1 atm). It is shown that the Stern-Volmer plots are downward curved and that the curvature can be explained either by a two-site model or by a negative deviation from Henry's law. A curve fitting procedure cannot distinguish between the two models.Using a frequency modulated blue LED and a fiber optic set-up, 0 2 partial pressure of 0.4 mbar could be detected. Oxygen can be determined at 200 mbar with a f 1.5% accuracy.