Contactless electroreflectance (CER) spectroscopy has been applied to investigate the optical transitions in Ga(In)NAs/GaAs quantum well (QW) structures containing Sb atoms. The identification of the optical transitions has been carried out in accordance with theoretical calculations which have been performed within the framework of the effective mass approximation. Using this method, the bandgap discontinuity for GaN0.027As0.863Sb0.11/GaAs, Ga0.621In0.38As0.954N0.026Sb0.02/GaAs, and Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs QW structures has been determined. It has been found that the conduction-band offset is similar to 50 and similar to 80% for GaN0.027As0.863Sb0.11/GaAs and Ga0.621In0.38As0.954N0.026Sb0.02/GaAs QWs, respectively. It corresponds to 264 and 296 meV depth QW for electrons and heavy-holes in GaN0.027As0.863Sb0.11/GaAs QW; and 520 and 146 meV depth QW for electrons and heavy-holes in Ga0.62In0.38As0.954N0.026Sb0.02/GaAs QW. In the case of the Ga0.6In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs step-like QW structure it has been shown that the depth of electron and heavy-hole Ga0.6In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW is similar to 144 and similar to 127 meV, respectively. (c) 2006 Elsevier B.V. All rights reserved.