The direct way to investigate the effect of an electric field F on the optical properties of a quantum well (QW) is to change the well width (Miller et al 1986 Phys. Rev. B 33 6976). Photoreflectance studies of a 100 Å and a 250 Å Ge/GeSi modulation doped QWs possessing 104 V cm−1 perpendicular fields are presented. This technique modulates the QW electro-absorption, which we compute using wavefunctions obtained from a 8 × 8 k ⋅ p calculation. They are shown to have a pronounced tunnelling into the bandgap in the 250 Å QW, in contrast to the 100 Å QW. Kramers–Kronig analysis is used to determine the corresponding changes in the QW refractive index, and overall reflection from the entire sample (including barriers) is obtained within a 2 × 2 matrix formalism. In a QW of width Lz, the Franz–Keldysh effect is expected to be important in an energy interval extending from near the bandgap Eo to Eo + eFLz. Its role in altering QW optical properties as a function of Lz is tested by comparing calculated and experimental spectra in this interval.