The optical reflectivity has been measured over more than four decades of photon energy on chemically well-defined single crystals of ScN, in addition to the electrical conductivity and the Hall effect. From a Kramers-Kronig analysis the dielectric functions have been derived. A plasmon-phonon coupling (plasmaron) has been observed in the far-infrared region which could be decomposed and the phonon part of which compared with a Raman measurement of the phonon density of states. The interpretation of the optical results in a two-band model together with other data confirms recent theoretical estimates of the electron and hole masses and of the charge-carrier density in this compound, which we find is a compensated semimetal.