Optical properties of diluteFe1−xSix(x⩽0.1)alloys were studied by spectroscopic ellipsometry in the 1.5–6 eV photon energy region. The absorption strength of the alloys is found to decrease in the entire energy region as the Si composition increases. The interband-transition edges of Fe at about 2.5 eV in the minority-spin bands and at about 5 eV in the majority-spin bands shift to lower energies as the Si composition increases. The decrease in the absorption strength is interpreted as due to a decrease in the density of states of the Fe host and also due to a reduction of conduction-electron concentration in the alloys. The shifts of the 2.5 and 5 eV interband-transition edges can be explained in terms of a perturbative variation in the band structure of Fe induced by its alloying with Si. ©1997 American Institute of Physics.