We report crystalline mixed-ligand copper complexes with phenanthroline and isocyanides with almost rnillesecond emission lifetimes that are efficient dioxygen sensors. The oxygen sensitivity of the prototype ([Cu(CN-xylyl)(2)(dmp)]tfpb, dmp = 2,9-dimethy1-1,10-phenanthroline; CN-xylyl = 2,6-dimethylphenylisocyanide; tfpb = tetralcis(bis-3,5-trifluoromethylphenylborate) is 38 times better than that of [Ru(phen)(3)]tfpb(2) (phen = 1,10-phenanthroline).