We have prepared polyelectrolyte brushes on silicon blocks in two steps: first, we graft polystyrene (PS) chains terminated by a trichlorosilane group, and then, we convert the grafted PS to poly(styrenesulfonate, sodium salt) by using a soft sulfonation reaction. The grafted layers are fully characterized by ellipsometry and infrared spectroscopy. Using neutron reflectivity, we determine the interfacial density profile of the polyelectrolyte brushes so formed in aqueous solutions. In pure water, the chains are strongly stretched because of the electrostatic repulsion between charged monomers; the mean thickness of the brush is proportional to the chain length and does not depend on the grafting density. Upon addition of salt, two regimes are observed. As long as the salt concentration is less than the inner concentration of counterions in the brush phi(ci); the added electrolytes have no effect. On the other hand, if the salt concentration exceeds c(ci), the polyelectrolyte layer shrinks as the ionic strength increases but never collapses.