Abstract A search for acid fluoride solutions which provided maximal fluoride uptake while causing minimal demineralization prompted a detailed study of solutions of sodium fluoride in acid sodium phosphate. Prolonged exposure of enamel to these solutions failed to cause etching. The fluoride was readily available and acquisition by enamel increased with increase in concentration and decrease in pH. There was appreciable penetration of fluoride into intact enamel after a 20 min exposure to a solution of phosphate-fluoride at pH 3. The deposited fluoride is present primarily as fluorapatite. Acid dissolution of powdered enamel was decreased after treatment with acid phosphate, and this effect was enhanced when fluoride was present in solution. These findings and the fact that acid phosphate-fluoride solutions are stable and cause no discoloration of teeth, suggest that they may be suitable for topical application treatments.