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Variable‐angle spectroscopic ellipsometry is used to determine the room temperature complex refractive index (n˜=n+ik)$\left(\right. \overset{˜}{n} = n + i k \left.\right)$of molecular beam epitaxy grown GaSb1−xBixfilms with x≤ 4.25% over a spectral range of 0.47–6.2 eV. By correlating to critical points in the extinction coefficient k, the energies of several interband transitions are extracted as functions of Bi content. The observed change in the fundamental bandgap energy (E0, −36.5 meV per %Bi) agrees well with previously published values; however, the samples examined here show a much more rapid increase in the spin‐orbit splitting energy (Δ0, +30.1 meV per Bi) than previous calculations have predicted. As in the related GaAsBi, the energy of transitions involving the top of the valence band are observed to have a much stronger dependence on Bi content than those that do not, suggesting the valence band maximum is most sensitive to Bi alloying. Finally, the effects of surface droplets on both the complex refractive index and the critical point energies are examined. Using variable angle spectroscopic ellipsometry, the complex refractive indices of GaSbBi alloys are determined as functions of wavelength and Bi fraction. From the films’ optical behavior, inferences into their underlying band structure are made. Specifically, the change in bandgap and spin‐orbit splitting energies are tracked as functions of Bi concentration.

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