The dielectric functions of equiatomic Ni-Ti alloys were measured by spectroscopic ellipsometry in the energy range of 1.5–5.4 eV at ∼423 and at ∼25 K. The peak at ∼2.26 eV in the B19′ (monoclinic structure) optical conductivity spectrum has a slightly larger magnitude than in the B2 (cubic CsCl structure) phase, while the shoulder at ∼3.5 eVbecomes weaker and almost indiscernible upon martensitic transformation. A new structure develops at ∼2.85 eV in the B19′ phase; however, it is also very weak. The band structures and the optical conductivity were calculated in both phases using the linearized-augmented-plane-wave method within the local-density approximation. k points near the Γ−X−M plane in the B2 phase and the corresponding k-points in B19′ phase contribute significantly to all three structures. The difference between the two spectra is due to the transitions between the folded-back bands from the B2phase because of the larger unit cell of the B19′ phase and the change in the electronic energy spectrum near the Fermi level. The overall optical properties of Ni-Ti alloys in the measured energy range are rather insensitive to the martensitic transformation because the states far from the Fermi level are mainly involved in the interband transitions.