Abstract We have studied optical transmission spectra of exfoliated α -RuCl 3 flakes as a function of thickness and strain. While we do not find intrinsic changes of the electronic structure as a function of thickness down to d ≈ 3 nm, a uniaxial strain dependence of the principal gap excitation and higher lying multiplet states is observed, which changes sign according to its compressive or tensile character. In particular, the gap decreases under tensile strain and the intensity ratio of the gap excitation, which is due to a d 4 triplet final state, with respect to the singlets at higher energy increases significantly. The d 4 triplet is involved in the superexchange interaction between neighboring Ru sites and is, therefore, sensitive to nearest-neighbor spin-spin correlations. The experimental observations indicate an enhancement of the Kitaev interactions relative to competing interactions like the Heisenberg exchange J under tensile strain.