The enhancement and shift of the fluorescence spectrum of 8-hydroxyquinoline (8-HQ) in heptane upon gradual additions of the anionic surfactant aerosol-OT(AOT) allowed us to show that monomeric 8-HQ was progressively associated to the polar heads via H-bonding. This association competes with dimerization of 8-HQ in heptane which has been revealed by us in a previous work using vapor pressure osmometry measurements. Similar competition has been qualitatively reported by others in the case of 7-aza-indole. In the present work, a comparison between the behaviors of 8- and 6-hydroxyquinoline was undertaken because the positions of the -OH and greater than or equal to N functions preclude dimerization in 6-hydroxyquinoline. This comparison resulted in the determinations of the binding constant of 8-NQ to AOT, K-ass = 860 +/- 30, and of the dimerization constant in heptane, K-dim = (7.0 +/- 1.5) x 10(7). Hence the dimers of 8-HQ in heptane, never reported before, turned out to be of outstanding stability.