The optical dielectric constants, epsilon(e) and epsilon(0), have been determined by ellipsometry for a monolayer of dodecanol on aqueous solutions of KBr. In the thin-film limit, ellipsometry yields only one independent parameter, eta, which is insufficient to define uniquely the three unknowns epsilon(e), epsilon(0), and the thickness of the monolayer, d. For a monolayer of dodecanol on water, d is known independently from X-ray diffraction. Varying the concentration of KBr changes the dielectric constant of the subphase and allows the calculation of both epsilon(e) and epsilon(0). The roughness of the interface and the depletion layer at the surface of the salt solution must be taken into account in the analysis of the ellipsometric data. We obtain values of epsilon(e) = 2.23 and epsilon(0) = 2.12 at an area per molecule of 20.9 Angstrom(2).