We use interband exciton transitions in parabolically graded quantum wells to measure the band offset at the InSb/AlxIn1-xSb interface. The method we use is based on similar studies in the GaAs/AlxGa1-xAs system but modified to reflect the strong nonparabolicity and strain of the InSb/AlxIn1-xSb system. We find a conduction band offset ratio of 0.62±0.04 for Al concentrations in the range 2%–12%. The observed lack of variation of the offset with Al concentration suggests a lack of strain dependence in the InSb/AlxIn1-xSb system for practical Al concentrations. © 2000 American Institute of Physics.