We describe the synthesis and the swelling behavior of a weak polyacid brush attached to a solid surface. Monolayers of poly methacrylic acid are generated by using self-assembled monolayers of an azo initiator and radical chain polymerization of methacrylic acid monomer at the surface of a planar substrate in situ. The thickness of the resulting surface-attached polyelectrolyte brush can be adjusted between 5 and 400 nm in the dry, collapsed state. The swelling behavior in water as a function of pH and salt concentration is investigated by multiple-angle null-ellipsometry. At high salt concentrations the brush thickness decreases due to electrostatic screening. At low concentration, however, an increase of thickness with ion concentration is found.