Stark shifts are calculated for the ground and first excited electron energ
y levels in single GaAs/Al0.35Ga0.65As quantum wells with non-abrupt interf
aces, which is a more appropriate picture for actual samples. The quantum-c
onfined Stark shift for the ground-state electron energy level decreases as
the non-abrupt interfaces become larger, but it is always negative. In str
iking contrast with this behaviour, the existence of non-abrupt interfaces
can change the sign of the quantum-confined Stark shift for the first excit
ed energy level in comparison to that calculated considering abrupt interfa
ces. These effects are shown to be stronger in the case of symmetric rather
than asymmetric non-abrupt interfaces.