Magnesium fluoride thin films were deposited on silica glass and single crystal silicon substrates by argon ion-beam assisted deposition (IBAD). The effects of argon ions impinging on the growing film on the optical and chemical properties of the single layers were investigated. Compared to MgF2 films produced by direct electron-beam evaporation, the films obtained by IBAD exhibited increased optical absorption and refractive indices, fluorine depletion and increased oxygen contamination. Optical data were analyzed and are discussed with regard to the chemical composition of the films.