Sum-frequency spectroscopy (SFS), ellipsometry, and surface tensiometry have been used to study the effect of halide counterion on monolayers of the cationic surfactant CTAX (CH3(CH2)(15)N+(CH3)(3); X-; X = F-, Cl-, Br-, and I-) at the air-water interface. The counterion was found to change the efficiency and effectiveness of the surfactant, both decreasing in order Br- > Cl (-) > F-. Aqueous solutions of CTAI were well below the Krafft point at room temperature, and CTAI therefore functioned ineffectively as a surfactant under ambient conditions. Adding salt in the form of 0.1 M KX was found to reduce the cmc but appeared to have little effect on the limiting area per molecule attained at the cmc, which increased from 44 Angstrom(2) for CTAB to 65 Angstrom(2) for CTAC and ca. 94 Angstrom(2) for CTAF. Neither SFS nor ellipsometry provided any firm evidence for specific effects of the halide ions on the structure of the surfactant monolayers. SFS was used to study monolayers of CTAB as a function of concentration. For surface coverages greater than 1 x 10(-6) mol m(-2) the mean tilt of the terminal methyl group from the surface normal was independent of the area per molecule. A decrease in the strength of the SF signal arising from the methylene groups at low surface coverages, which would be expected for molecules lying flat on the surface, was not observed. In the course of the analysis of the ellipsometric data, the contribution of the double layer to the coefficient of ellipticity was evaluated and was shown to be linear in the surface coverage and insensitive to the thickness of the double layer.