We apply a lattice-based self-consistent mean-field (SCF) theory in order to investigate a dense bidisperse polymer brush. The system studied contains polyisoprene chains of 485 and 970 repeat units. We compare our results to neutron reflectivity experiments recorded at two different average surface densities of the grafted chains (0.323 and 0.455 nm(-2)). Vie investigate the inner structure described by the bottom region, formed by a mixture of long and short chains, and the top region composed only of segments of the longer chains. The inner 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. The agreement between the experimental results and our SCF results is very good. We also systematically study the structure of the bidisperse brush for various values of the molar fraction of the longer chains, of the length ratios and of the overall surface densities.