The optical properties of TiC", TiN", VC", and VN" in the energy range 1.5 to 40 eV were determined with the use of electron-energy-loss spectroscopy. The optical joint density of states deduced from a Kramers-Kronig analysis were compared with joint densities of states calculated from selfconsistent Gaussian-LCAO band-structure data. Good agreement between theory and experiment was obtained. Characteristic differences among the spectra of the particular compounds are discussed in terms of different electronic properties deduced from band-structure calculations. The influence of nonmetal vacancies on the optical properties was investigated.