Optical-absorption measurements on n- and p-type Mg2Ge single crystals were made at wavelengths between 1.5 and 25 μ at temperatures between 4.2 and 300°K. The absorption edge in p-type crystals arises from allowed indirect transitions. The energy gap is 0.570 eV at 0°K and the phonon involved has an energy of 0.025 eV. At higher temperatures the temperature dependence of the energy gap is -1.8×10−4 eV/deg. At longer wavelengths p-type crystals exhibit absorption bands interpreted as being caused by transitions between three valence bands at the center of the Brillouin zone as in germanium. The spin-orbit splitting of these bands is 0.20 eV and the ratio of heavy-hole mass to light-hole mass is 6±1. The n-type samples have an absorption band at 0.58 eV (0°K) of uncertain origin, and λ2.5 free-carrier absorption, indicating optical-mode scattering.