Abstract The reflectivity at normal incidence of single crystals of Na +C222 · Na − was measured from 350 to 2500 nm in a microreflectivity apparatus. The reflectivity spectrum shows a single peak at 630 ± 10 nm (1.97 eV) with a peak reflectivity of up to 60%. No rise in the reflectivity was found toward longer wavelengths. This absence of a plasma edge confirms that the concentration of conduction electrons is smaller than 1.8 × 10 20/cm 3 as expected from the low electrical conductivity of this sodide. The absorption spectrum of thin solid films of Na +C222 · Na −, formed by vapor deposition, was measured in situ in a vacuum evaporator. The absorption peak was at 650 ± 10 nm (1.91 eV), with a full-width at half-maximum of 0.37 eV. The reflectivity and absorption data were used together to estimate the indices of refraction and absorption and the components of the complex dielectric constant as a function of wavelength.