Excitonic photoluminescence in a transverse electric field has been studied in strained Si1−xGex/Si type‐I quantum wells (QWs) in the weak field regime. With increasing electric field, a steady blue‐shift of the free exciton emission energies has been observed, thereby suppressing the anticipated downward shift due to a quantum confined Stark effect. It is revealed that a field‐driven decrease of the exciton binding energies gives a good account of the experimental blue‐shift for a wide range of well widths, which is also in good agreement with variational calculations. We also find that the extremely small conduction band offset characteristic of stained Si1−xGex/Si type‐I QWs is responsible for the absence of the Stark red‐shift and the mixed dimensionality. ©1996 American Institute of Physics.