The sputter‐induced optical emission technique has been used to study the classical problem of the exchange of fluorine and hydroxyl ions in a solid. The hydroxyl ions were measured in the form of hydrogen atoms and detected by the characteristic hydrogen emission line at 6563 A˚ while the fluorine atoms were detected at 6902 A˚. Measurement of the optical intensities as a function of sputtering time produced the H and F depth profiles in GdF3films.