We investigated the influence of thermal interdiffusion on the band structures of Si1−xGex /Si single quantum wells SQWs with a well width of 125 Å and Ge concentration x=0.3. Rapid thermal annealing with the temperatures of 750 and 800 °C leads to substantial interdiffusion indicated by a significant photoluminescence PL blueshift as large as 46 meV. The effect is modeled by a Si–Ge atomic interdiffusion at the heterointerface. The band structures and optical transitions of QW after interdiffusion were calculated based on an error function distribution and the 6+2-band k· p method. The diffusion lengths of the intermixing process are deduced from the PL shift. The thermal dependence of the interdiffusion coefficients follows the Arrhenius law. An activation energy Ea for interdiffusion of 2.75 eV is obtained. Our investigation indicates that the 6+2-band k· p formalism is valid for interdiffused Si1−xGex/Si QWs.