A 77.8 MHz surface acoustic wave (SAW) device has been used to characterize the deposition of monolayers of an indigo dye substituted with two hydrophobic alkyl chains. The dye forms a monolayer with two distinct slopes, whereas a mixture of the dye with cadmium arachidate forms a monolayer typical of fatty acids. The deposition of mixed layers is homogeneous, which also can be concluded from UV-visible spectra. The surface mass density associated with each deposited monolayer agrees within 10% with that calculated from the molecular surface area. The SAW device reveals the necessity of allowing the monolayers to dry before successive deposition.