The synthesis of molecularly thin poly(p-styrenesulfonate) monolayers as an example for a negatively charged polyelectrolyte brush covalently attached to a planar solid surface is described. The polymer monolayers are generated directly at the surface of the substrate by using self-assembled monolayers of an azo initiator and radical chain polymerization of p-styrenesulfonate ethyl ester (ESS) monomer in situ. This "grafting from" approach yields surface-bound poly(p-styrenesulfonate ethyl ester) (PESS) molecules with high molecular weights and with high graft densities of the attached chains. The PESS monolayers can be transformed into a charged poly(p-styrenesulfonate) (PSS) monolayer through a polymer-analogous saponification reaction under mild conditions, and the reaction can be carried out to quantitative conversion. The thickness of the polyelectrolyte brush can be controlled from 2 to more than 35 nm in the dry, collapsed state.