The swelling behavior of poly(methacrylic acid) (PMAA) brushes covalently attached to solid planar surfaces in aqueous solutions containing different monovalent salts was investigated with multiangle null ellipsometry. The polymer chains were grown in situ from the surface by surface-initiated polymerization using a surface-attached initiator ("grafting from" method). The swelling of the PMAA brushes was measured as a function of pH value, concentration of added low molecular weight salt, and the grafting density of the surface-attached polymer brushes. At pH values between 4 and 10, the brush thickness initially increases and then decreases with increasing concentration of added salt; i.e., a maximum of brush thickness appears at a medium salt concentration. The influence of the chemical nature of different monovalent co- and counterions on the swelling behavior of the brushes is studied.