AbstractThe strong increase of the dielectric constant observed at a critical antimony concentration in semiconducting bismuth‐antimony alloys is interpreted in terms of a purely electronic interband polarization enhanced by vanishing of the thermal energy gap. The dominant indirect transitions between conduction and valence band states near the L‐ and T‐points of the Brillouin zone are considered as induced by the short‐range scattering potential due to the chemical disorder of the alloy. Reasonable system parameters allow for a satisfactory fit to the experimental data.