The conductivity tensor of polycrystalline Pt3X (X=Mn,Co) was determined between 1.6 and 5.2 eV. Samples were arc melted, mechanically polished, and annealed at 500°C for 1 h in Ar. The complex dielectric function was measured from 1.3 to 5.2 eV at room temperature with a rotating analyzer ellipsometer. The magneto-optic Kerr effect was studied between 10 and 293 K in magnetic fields up to 3 T. We used the tight-binding linear-muffin-tin-orbital method in the local spin-density approximation to determine the band structure, density of states, and optical conductivity. Including an empirical quasiparticle self-energy and a lifetime broadening yields good agreement of experimental and calculated spectra.