A two-dimensional first-order phase transition has been studied-by sum-frequency vibrational spectroscopy in mixed monolayers of dodecanol and alkyltrimethylammonium bromide surfactants (C(n)TABs) at the air/water interface, with n = 12, 14, and 16. At low temperatures (<19 degrees C); mixed monolayers adsorbed from solutions of 10 mu M dodecanol and C(n)TAB at two-thirds of their critical;micelle concentrations are densely packed and highly conformationally ordered. The density of the: monolayer is comparable to that of a monolayer of dodecanol in its low-temperature crystalline phase. At a fixed temperature, which increases with C(n)TAB chain length, the monolayer melts to a phase that is less dense and more conformationally disordered. Specific deuteration experiments were carried out to determine the composition of the monolayer and the conformational order of the two components. The ＇solid＇ phase contains dodecanol and C(n)TAB in a ratio of approximately 3:2, and the ＇liquid＇ phase contains equal amounts of the two components. As the chain length of the C(n)TAB increases, the number of conformational defects in the dodecanol molecules, as measured by the line strength of the symmetric methylene stretch, decreases in both phases. The, C(n)TAB in the mixed monolayer is more disordered than the dodecanol but much less disordered than in a pure C(n)TAB monolayer in the absence of dodecanol. Frequency shifts of the symmetric methyl stretch with changes in C(n)TAB chain length can be understood in terms of the changing environment of the dodecanol and C(n)TAB methyl groups.