We report a study of the compression of monolayers of monodisperse spherical polystyrene particles at the interface between aqueous surfactant solutions and octane. The particle size (2.6 pm diameter) was selected so that direct in situ microscopic observation of the monolayer structure could be made during lateral compression and "collapse". Monolayers have been formed on a miniature Langmuir trough placed on a microscope stage. Our study has focused on (a) the relationship between the monolayer collapse pressure, Pi (col), and the interfacial tension, gamma*, of the oil/water interface in the absence of a particle monolayer and (b) the mode of monolayer "collapse" at high surface pressure. Interfacial tensions gamma* have been adjusted (in the range 50-4 mN m(-1)) by addition of surfactants over a range of concentration. We find that the monolayer collapses by buckling (folding) when the surface pressure is equal to the surface tension of the oil/water interface, Particle promotion out of the interface is not observed experimentally.