We have used anomalous small-angle X-ray scattering (ASAXS) to investigate the spatial correlation of counterions with a macroion consisting of polyelectrolyte brushes. The spherical polyelectrolyte brush is composed of a solid poly(styrene) core of 68 nm radius with a dense grafting of linear poly(acrylic acid) chains and rubidium. counterions. ASAXS allows one to evaluate the spatial distribution of counterions by exploiting the selective variation in contrast near the absorption edge of rubidium. The deduced spatial distribution of the counterions shows that they are closely correlated to the polymer chains of the macroion. The correlation between ions and polymer chains in these brushes is much stronger than that found in linear polyelectrolytes.