Abstract Mixing between Γ and X valleys of the conduction band in GaAs–Ga 1– x Al x As quantum wells is investigated taken into account the effect of applied hydrostatic pressure. This effect is introduced via the pressure‐dependent values of the corresponding energy gaps and the main band parameters. The mixing is considered along the lines of a phenomenological model. Variation of the confined ground state in the well as a function of the pressure is reported. The dependencies of the variationally calculated binding energy of a donor impurity with the hydrostatic pressure and well width are also presented. It is shown that the inclusion of the Γ–X mixing explains the non‐linear behavior in the photoluminescence peak of confined exciton states that has been observed for pressures above 20 kbar. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)