A method is described for the determination of sub-microgram amounts of inorganic fluoride. Diffusion on to a thin layer of solid alkali is used to separate fluoride from any contaminants. The diffused fluoride may be determined with an ion-selective fluoride electrode or by spectrophotometric methods. Phosphate and sulphate do not interfere. The interference of halides, carbonate, nitrate and acetate is discussed. Organic material was found not to interfere with the recovery of fluoride except in determinations involving human serum. High-temperature ashing of organic samples was found to lead to loss of fluoride. When used with the ion-selective electrode the method is applicable over the range 0·05 to 200 µg of fluoride.