The photoreflectance (PR) spectroscopy of the single quantum well InxGa1−xAs/GaAs system has been measured at various temperatures. The selection rules for the interband transitions are &Dgr;n=0, wherenis the quantum number of thenth subband in the quantum well. The symmetry forbidden transitions (&Dgr;n≠0), such as 12H(wheremnHdenotes transition between themth conduction tonth valence subband of heavy hole), were often observed in the experiments and it was attributed to the existence of the built‐in electric field in the quantum well. In this work, we change the strength of the built‐in electric field by varying the temperatures of the samples. By varying the temperatures of the samples, the strength of the field can be changed by the effect of photo‐induced voltages. The measured ratios of the intensities of 12Hto 11Htransitions decrease as the temperatures are lowered. Therefore, the existence of the built‐in electric field may account for the observations of the symmetry forbidden transition 12Hin the experiments. ©1995 American Institute of Physics.