We have studied by means of low temperature photoluminescence (PL) and photocurrent spectroscopy the effects of an external electric field on the excitons in GaAs quantum wells confined between GaAlAs. Increasing the field causes a Stark shift of the excitons toward lower energies with a simultaneous quenching in the PL intensity. At moderate fields, we find very good agreement (better than 0.5 meV) between the light- and heavy-hole exciton energies obtained by PL and photocurrent measurements. A significant deviation in energy of the PL relative to the photocurrent is observed at high fields, manifesting the increase in the contributions of impurity-bound excitons to the PL lineshape. A detailed PL study of the Stark shift as a function of well thickness has also been performed. The results show an increasing Stark shift with increasing well thickness, amounting to 110 meV for a 230 Å-wide well at a field of 105 V/cm. For very wide wells (∼ 1000 Å) the behavior of bulk GaAs is recovered: the excitons become ionized before large Stark shifts can be observed. Variational calculations have been carried out and shown to account for the experimental observations of both the Stark shift and the quenching of the PL. In this light, we will discuss the mechanisms governing the optical properties of quantum wells under an external electric field. © 1987.