Magnesium fluoride films have been deposited on quartz and silicon substrates by direct electron beam evaporation. The structure, composition, and mass density of films were investigated as functions of the deposition temperature ranging from ambient temperature to 300 degrees C. The composition of films determined by infrared spectroscopy, and the residual stresses in films calculated from the change of the radius of curvature of Si substrates, were studied as functions of the aging time of films in room air and deposition temperature. The aging behavior is analyzed and discussed in connection with the microstructure and mass density of films.