Vibrational sum frequency spectroscopy (SFS) has been used to study monolayer films adsorbed from aqueous solutions of didodecyldimethylammonium bromide (DDAB) and dodecan-1-ol (C₁₂OH) onto a planar model hydrophobic surface. The DDAB film adsorbed from pure solution is conformationally disordered and becomes steadily more disordered as the solution concentration decreases or the temperature increases. In contrast, the C₁₂OH film adsorbed from saturated solution produces a densely packed monolayer which exhibits a first-order phase transition near 313 K. The difference in conformational orders is thought to arise from the fact that DDAB has a large positively charged head group and two non-parallel alkyl chains which prevent close-packing in the monolayer. In solutions containing both DDAB and C₁₂OH coadsorption occurs when the DDAB concentration is at least 1/10 of its critical aggregation concentration (c.a.c.) and persists well above the phase-transition temperature of C₁₂OH. For DDAB concentrations below 1/10 c.a.c. the adsorbed film contains only C₁₂OH. For DDAB concentration equal to 5 c.a.c. the adsorbed film contains a fractional C₁₂OH coverage of ca. 1/5. Experiments using deuteriated dodecan-1-ol (d-C₁₂OH) show that DDAB in coadsorbed films is more conformationally ordered than the pure DDAB monolayer.