Photoluminescence and photomodulated transmission measurements on In 0.2Ga0.8As/ GaAs/Al0.3Ga0.7As modulation doped pseudomorphic single quantum wells are presented. Photomodulated transmission spectra at low temperatures showed sharp lines that are separated with respect to the luminescence peaks due to the Stokes shift. From the Stokes shift we estimated the Fermi energy and the two-dimensional electron gas density. The obtained results are in good agreement with Shubnikov-de-Haas data. The temperature dependence of the optical spectra was also investigated. The photomodulated transmission technique is shown to be a good tool to evaluate the electronic properties of modulation doped single quantum well structures, including an estimation of the Fermi energy and the two-dimensional electron gas density.