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The nanostructure of a spread monolayer of diblock copolymers of poly(hydrogenated isoprene) and poly(styrenesulfonate) at the air/water interface were studied by in situ X-ray reflectivity as a function of the brush density and salt concentration. When the monolayer was compressed beyond the “critical brush density”, its nanostructure changed from a flat, adsorbed “carpet” layer to a “carpet + brush” structure. The critical brush density was found to be about 0.12 nm^-2, independent the brush length and salt concentration under a low-salt condition. The brush formation behavior was considered to be controlled by an electrostatic interaction between polyelectrolyte chains rather than by a steric hindrance. This might be because the distance between the chains at the critical point is rather long and also because of the effect of the salt on the critical brush density. The critical brush density increased at higher added salt …