Using GaAs based quantum well infrared photodetectors (QWIPs) with either GaAs or InGaAs wells, we experimentally investigate the accuracy of the polarization selection rule for conduction band intersubband transitions. We employ a device structure and a light coupling geometry where the parasitic light scattering is negligible. The experiments imply that the selection rule is followed to an accuracy of 0.2&percent; for a 8.1 &mgr;m QWIP with GaAs wells; this degrades to 3&percent; for a 4.6 &mgr;m QWIP withIn0.1Ga0.9Aswells. ©1998 American Institute of Physics.