The electronic structures of AuGa2 and PtGa2 have been studied with use of spectroscopic ellipsometry and the dielectric functions have been determined in the 1.2–5.5-eV region. Both compounds show interband absorption at low photon energies (<1.3 eV). The interband absorption for AuGa2 is strong at about 2 eV while that for PtGa2 shows a broad structure in the range 2.5–4.5 eV, with a shoulder at 3.3 eV. The observed interband features in ε2 can be interpreted in terms of self-consistent relativistic band-structure calculations using the linear augmented-plane-wave method. The interband contribution to the imaginary part of the dielectric function εb2 has been calculated including the effect of the electric dipole matrix elements. The overall agreement is good between the band calculations and the ellipsometry results in the 1.2–5.5-eV region for both the magnitude and the positions of the structures. Below 1.2 eV the calculational results for both compounds show interband absorption, which is also qualitatively suggested by the ellipsometry results.