A new approach for the study of grafted polyelectrolytes is investigated: monolayers at the air/water interface of block copolymers consisting of a liquid hydrophobic and a polyelectrolyte block. The lateral interactions between the molecules is extremely long-ranged; a lateral pressure can be measured, while the grafting density is varied by a factor of 10. Furthermore, the isotherms show a phase transition. With specular X-ray reflectivity the segment density profile along the isotherm in the compressed phase is determined. The hydrophobic block shows features characteristic of a melt: constant electron density and a linear relationship between length and grafting density. In contrast, the length of the polyelectrolyte block amounts to 66% of the contour length and remains constant while the grafting density is varied by a factor 2. These features are typical for an osmotic brush phase.