Surface light scattering has been used to investigate properties of pure hexadecyltrimethylammonium bromide (C(16)TAB) and mixed dodecane-C(16)TAB monolayers at the air-solution interface. Significant changes are observed, the most notable being a sign change in the dilational viscosity epsilon ’ from negative to positive (with dodecane). This behavior is consistent with an increase in adsorption barrier as proposed by Hennenberg et al. (J. Colloid Interface Sci. 1992, 150, 7).