The function of dipalmitoyl phosphatidylcholine (DPPC) Langmuir monolayers was studied after exposure to various concentrations of 200 nm carboxyl-modified polystyrene particles by a combination of surface pressure and surfactant microstructure studies. The presence of particles in the subphase at the lowest concentrations tested (10⁻⁵ to 10⁻⁴ g/L) did not influence the π–A isotherms. However, at the highest concentration (10⁻³ g/L), changes in the hysteresis areas of the isotherms were observed. The formation of LC domains during compression was significantly altered by the presence of the particles, resulting in the formation of smaller but more numerous domains. Fluorescence and atomic force microscopy images suggested that particles remained in the subphase but were closely associated with the condensed domains.