The room-temperature reflection spectra of Mg2Si, Mg2Ge, and Mg2Sn have been measured at wave-lengths between 20 and 50μ. Reststrahl reflectivity peaks were observed at 36 and 45 μ for Mg2Si and Mg2Ge, respectively, and a rising reflectivity in Mg2Sn is estimated to peak at about 54 μ. The occurrence of peaks indicates that these compounds are partly ionic. The reflection spectra are more like those of the alkali halides than those of the III-V semiconducting compounds. If a simple damped harmonic oscillator is used to describe the reststrahl mode, the three Mg compounds have the same force constant for this mode. Values of dielectric constants and effective charges are estimated for Mg2Si, and are used to evaluate a previous analysis of electron mobility in this compound. Effects of free carriers on the reflectivity of Mg2Ge indicate that the effective masses are in rough agreement with the results of transport measurements.