The accumulation of oxygen at GaAs/AlGaAs interfacesgrown by molecular beam epitaxy has been established by secondary ion mass spectrometry profiling of GaAs/AlGaAs multilayerstructures. An enhanced oxygen peak was observed at the boundary between GaAs and Al x Ga1−x As layers with x=0.35 and x=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 multilayerquantum wellstructures and for the difference between normal and inverted interface high electron mobility devices.