TheG1c–G15vbandgap of the AlxGa1–xAs compound semiconductor withxranging from 0.06 to 0.87 was determined by fitting their Schottky barrier spectral response with a device model. Since the spectral response near the AlxGa1_xAs bandgap is proportional to its absorption coefficient, the bandgap could also be obtained by direct extrapolation of the square response versus photon energy curve to zero response. Both results are shown to be in good agreement. Using samples withxlarger than 0.48, we could also observe additional responses originated fromG15vtoX1candL1cindirect transitions. By using least‐squared fit we obtained not only theXband but also theLband energies. The electrolyte electroreflectance method was also applied to measure the direct bandgap of those samples withxranging from 0.06 to 0.59, and gave similar results.