Mixed self-assembled monolayers (SAM) of alkyl phosphonic acids and C-60 functionalized octadecyl phosphonic acids (C60C18-PA) are deposited on alumina substrates from solution and are shown to form well-ordered structures with an insulating layer of alkyl chains and a semiconducting layer that comprises mainly C-60. Such an ordered structure is a necessity for the application of SAMs in organic transistors but is difficult to obtain since C60C18-PA without additional support do self-assemble in dense packaging but not in a well-ordered fashion. To avoid disordering of the SAM and to gain a better control of the interfacial properties we have investigated the stabilizing effects of fluorinated dodecyl phosphonic acids (FC12-PA) on the C60C18-PA monolayer. Vibrational sum-frequency (SFG) spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, and electrical measurements were applied to study the mixed monolayers. Here, we make use of the differently labeled PA to determine surface coveravs and molecular properties of the two species independently. Adsorption of FC12-PA gives rise to vibrational bands at 1344 cm(-1) and 1376 cm(-1) in SFG spectra, while a pronounced vibrational band centered at 1465 cm(-1) is attributable to C-60 vibrations. The coexistence of the bands is indicative for the presence of a mixed monolayer that is composed of both molecular species. Furthermore, a pronounced maximum in SFG intensity of the C-60 band is observed for SAMs, which are deposited from solutions with similar to 75% C60C18-PA and similar to 25% FC12-PA. The intensity maximum originates from successful stabilization of C-60 modified C60C18-PA by FC12-PA and a significantly improved molecular order. Conclusions from SFG spectra are corroborated by electric measurements that show best performance at these concentrations. Our results provide new information on the morphology and composition of C-60 modified SAMs and establish a route to fabricate well-defined layers for molecular scale organic electronics.