The accumulation of oxygen at GaAs/AlGaAs interfaces grown by molecular beam epitaxy has been established by secondary ion mass spectrometry profiling of GaAs/AlGaAs multilayer structures. An enhanced oxygen peak was observed at the boundary between GaAs and AlxGa1−xAs layers withx=0.35 andx=1 when the binary layer is deposited on top of the ternary layer. The segregation of oxygen may be a contributing factor responsible for the lower luminescence reported in the first GaAs well of multilayer quantum well structures and for the difference between normal and inverted interface high electron mobility devices.