Abstract The cyclotron emission technique is applied to a number of GaAs/GaAlAs heterostructures and multilayers with mobilities ranging from 4 × 10 4 to 2 × 10 6 cm 2/V·s. The emission spectra consist in addition to a line at the cyclotron resonance position of a broad line at the higher energy side of the spectrum. We assign this spectral part of the emission to an impurity shifted cyclotron resonance. From the determined size of the binding energies in the order of 2 meV the impurities are assumed to be located close to the interface between GaAs and GaAlAs. The impurities are most likely Si donors which have diffused from the high doped GaAlAs region. The integral intensity of the impurity associated line is found to be directly correlated with the sample mobility. A number of 10 9 impurities per cm 2 is found for samples with mobilities of about 4 × 10 5 cm 2/V·s.