The dielectric function (DF) of a modulation doped (001) GaAs/AlGaAs (Al content of x = 0.33) single quantum well structure containing a two-dimensional electron gas (2DEG) was calculated as a function of 2DEG density and temperature, taking into account the nonparabolicity of the valence band and many-body effects. The DF was used to simulate electroreflectance and photoluminescence excitation spectra. The results were found to be in a good agreement with experimental data at T> 80 K and 2DEG concentrations of more than 2 X 10(11) cm(-2). (C) 2003 Elsevier B.V. All rights reserved.