The refractive index difference at the GaAs lasting wavelength of[inverted lazy s] 9000 Åhas been experimentally determined, from the measured beam profile, for a range of (AlGa)As&sngbnd;GaAs heterojunctions of practical interest. Good agreement is found with values calculated from the change in band gap with increasing Al in (AlGa)As. We have also shown by direct measurement that the loss of electron confinement atp+‐pheterojunctions follows a thermally activated process with activation energy equal to the difference in band gap at the heterojunction.