The volume fraction profile of long chains within a monolayer of a dense bidisperse polymer brush is investigated using neutron reflectivity. Monolayers of polymer brushes are prepared by spreading equimolar mixtures of short nondeuterated and long perdeuterated polyisoprenes with quaternary ammonium headgroups onto the surface of a Langmuir trough filled with H2O or H2O/D2O mixtures. The chain lengths of the shorter and longer chains are approximately 485 and 970 repeat units, respectively. The neutron reflectivity of the monolayer-covered water surface is recorded at two different mean areas per polymer chain (220 and 310 Angstrom(2)). The analysis of the reflectivity data yields a total brush thickness that is proportional to the surface concentration and is in accordance with the bulk density of the polymer. The inner structure can be described by a stratified model: The bottom region is formed by a mixture of long and short chains. On top of this region is a second part composed only of segments of the longer chains. The content of long chains in the bottom region is higher than that predicted by analytical self-consistent field (ASCF) theories. Thus, the parts of the long chains within the mixed region are less stretched than predicted by ASCF theories. The structure approximately scales with the total brush height, although small deviations occur. The width of the transition zone between the two regions seems to be independent of the brush height.