We have determined the alloy composition of modulation doped AlxGa1−xAs/GaAs heterostructure by Raman scattering and photoreflectance spectroscopy at room temperature. We also demonstrated that Franz‐Keldysh oscillations in photoreflectance spectroscopy can be used to evaluate the band gap of semiconductor heterostructure. The band gap measured by Franz‐Keldysh oscillations of photoreflectance spectrum is 1.865 eV. The alloy composition calculated from the band gap is 0.30. From the frequency positions of ‘‘GaAs‐like’’ and ‘‘AlAs‐like’’ phonon modes of the Raman scattering data, the value of the alloy composition was evaluated to be 0.29 which is in good agreement with the photoreflectance result. Both values also agree with the target composition in the molecular beam epitaxy growth and the target composition is equal to 0.30. ©1995 American Institute of Physics.