The surface pressure-area (pi-A) isotherms of dioctadecyldimethylammonium chloride (2C(18)DAC) and dioctadecyldimethylammonium cinnamate (2C(18)DA.Cin) were measured, and their time dependence was examined. The surfaces of Langmuir-Blodgett films of 2C(18)DAC and 2C(18)DA.Cin were observed by atomic force microscopy. The effect and location of cinnamate anions against molecular organization on Langmuir monolayers of 2C(18)DA are discussed. After preparation, cinnamate ions bind on the hydrophilic surface of the 2C(18)DA monolayer, by forming ion pairs with 2C(18)DA cations and by directing aromatic groups toward the interior of the aqueous subphase. When the Langmuir film is maintained for long periods at a constant surface pressure, cinnamate ions are intercalated into the monolayer, as aromatic groups penetrate into the hydrophobic interior of the monolayer.