pH-Sensitive adsorption of polymer particles bearing poly[2-(dimethylamino)ethyl methacrylate] hairs at the air water interface was investigated using a surface tensiometer, a Langmuir Blodgett trough, and an X-ray reflectometer. We clarified that the particles are adsorbed at the interface at basic pH; by contrast, at acidic pH, only a small number of particles are adsorbed, whereas the majority are dispersed in the water phase. X-ray reflectometry analysis revealed that a particle monolayer was formed at the air water interface, which was packed densely under increasing surface pressure, as determined by the electron density profile change. The contact angles of the particles at the air water interface were estimated to be 29 and 34 at pH 3 and 10, respectively, by direct-visualization of the air water interface position of the particles using a polycyanoacrylate trapping method.