Interaction of a positively charged dioctadecyldimethylammonium bromide monolayer with three different single-charged hydrophobic anions, namely, hexafluorophosphate (HFP), picrate, and tetraphenylborate (TPB), was studied by surface pressure, surface potential isotherm, and ellipsometric techniques. Two models of monolayer-ion interaction were considered : adsorption to the head groups and incorporation into the monolayer. It was shown that the nature of ion-monolayer interaction depends on the hydrophobic properties of ions. The least hydrophobic HFP anion adsorbs to the head groups of the monolayer. This leads to a moderate decrease of the surface potential and more close packing of the monolayer. Ellipsometric data as well agree with the adsorption model. More hydrophobic picrate and strongly hydrophobic TPB anions incorporate into the monolayer. It reveals a drastic drop of the surface potential, which is due to the formation of an additional double electric layer inside the monolayer. Variations of phi-A, pi-A, and ellipsometric isotherms support the incorporation model in this case. The experimental data show that hydrophobic ions cause much more pronounced changes in Langmuir monolayers than those produced by the usual (hydrophilic) ions. The mixed monolayer with a high content of large hydrophobic particles has its own particular mechanical and electrical properties, phase behavior, etc., which are quite different from those of the precursor monolayer.