We have investigated, theoretically and experimentally, the reduced and optical bandgap shift of Si-doped AlXGa1-XAs alloys as a function of both the Al composition and the Si concentration. The calculations were carried out within a framework of the many particle random phase approximation with the Hubbard local-field correction, considering electron populations in the conduction minima located at the Γ, X and L-points of the Brillouin zone. The experimental data have been obtained by means of photoluminescence spectroscopy. The theoretical predictions are found to be in good agreement with the experimental results.