We have observed optical transitions between the first subbands of the conduction band and the split‐off valence band in GaSb/AlSb quantum well structures. The well width dependence of the emission energies is traced to quantization in the conduction band and in the split‐off band. By comparison with data for the band‐edge transitions the effective &Dgr;0gaps is the quantum wells are determined. Contrary to previous calculations the &Dgr;0gap energies are almost independent of the well width.