Calculations of electroabsorption in extremely shallow quantum wells are performed, accurately incorporating mixing of different subband pairs due to the Coulombic interaction. As the AlAs mole fraction is varied in the barriers of a 100 Å wide Al x Ga 1−x As/GaAs square quantum well, a transition from red to blueshift of the absorption edge with applied electric field occurs at x ∼ 0.003. In a 20 Å wide square well, which more strongly confines the excitons, the redshift of the absorption edge is still observable at low electric field strength in the simulated absorption spectra, even for AlAs mole fractions as low as ∼0.001. However, on increasing the strength of the applied electric field the blueshift becomes apparent. It is demonstrated that inclusion of Coulombic coupling between different subband pairs, particularly between confined states and quasi-continuum states, is essential for a correct prediction of the absorption edge behaviour in extremely shallow quantum wells.