Fluoride ion depresses the absorption of magnesium in the air-coal gas flame, the change in absorbance of the magnesium resonance line at 2852 Å being proportional to the fluoride concentration over the range 0.2–20 µg/ml (10−5-10−3M). In the absence of interfering ions, principally sulfate and phosphate, the effect can be used to determine fluoride ion in this concentration range. An alternative, though somewhat less sensitive, technique is based on the enhancement of zirconium absorption by fluoride ion in the nitrous oxide-acetylene flame; this allows the determination, with a high degree of freedom from interference, of fluoride ion over the range 5–200 µg/ml (2.5 × 10−4-10−2M). In the presence of phosphate, which interferes with the zirconium method, a similar enhancement of titanium absorption can be used for fluoride determinations in the range 40–400 µg/mI (2 × 10−3-2 × 10−2M). These atomic absorption methods for fluoride determinations have been found to be considerably more rapid than more classical methods and in many cases can be used directly without prior separation of fluoride. Their application has been demonstrated in the analysis of a wide range of inorganic fluorides.