A study is presented of compositional modulations (i.e., so‐called banding) in heteroepitaxial strained layers of nominally uniform Ge0.1Si0.9grown by molecular‐beam epitaxy (MBE). The oscillatory nature of the modulations is studied by transmission electron microscopy (TEM) techniques and the results show that the contrast modulations have a periodicity of ∼23 nm when 1‐μm‐thick epilayers are grown on a rotated substrate. For growth on an unrotated substrate, contrast modulations are still observed but they are less regular. It is further shown that the electron microscope image features are dominated by strain contrast and not structure factor contrast. The presence of strain is consistent with there being a small change in lattice parameter between adjacent bands of material, together with elastic relaxations afforded by the proximity of the thin‐film surfaces. It is concluded that these variations in the lattice parameter are due to the compositional modulations. Limits on the magnitude (Δx) of the compositional modulations are assessed from considerations of diffraction contrast theory (giving an upper limit) and elasticity theory (giving a lower limit). The upper and lower limits for the compositional modulations, assuming an average 9 at. % Ge–Si epilayer, are ±1 at. % and ±0.25 at. % Ge, respectively. The origin of these compositional modulations is thought to be associated with equipment instabilities or a metastable miscibility gap.