Abstract—The phosphorescence of 1‐bromonaphthalene and 1‐chloronaphthalene is readily observable in nitrogen purged aqueous solutions containing ß‐cyclodextrin. Addition of acetonitrile increases both the phosphorescence intensity and lifetime. The quenching of halonaphthalene phosphorescence in aqueous solution by nitrite is substantially inhibited upon addition of ß‐cyclodextrin, as a result of a guest‐host complex. The rate constants for formation and dissociation of the l‐bromonaphthalene/ß‐cyclodextrin complex are evaluated from an analysis of the dependence of phosphorescence lifetimes on nit