Saslaw and Gaustad1 have recently suggested that interstellar dust may consist of small diamond particles. This suggestion can be verified if the optical properties of such particles can be determined. The optical constants of bulk diamond have been measured by Phillipp and Taft2 over the entire waveband of astronomical interest, ∼2 micron–1000 A. The real and imaginary parts of the complex refractive index (n, −k) are plotted in Fig. 1 as functions of the wave number. Using these data the extinction curves for diamond particles of various radii were calculated from the Mie theory. In Fig. 2 the extinction efficiency Qext is plotted as a function of 1/λ for the two particle radii, a=0.05 micron and a=0.10 micron. Both curves start off from the origin as λ−4, reach maxima at rather sharply peaked resonances and then wiggle asymptotically about a value close to ∼2.5. The wavelengths of the maxima could be shifted by varying the particle radius, but it is clear that the same general trends in the Qext curve persist for most sizes of interest.