A new extraction–spectrophotometric method has been developed for the determination of trace amounts of phosphorus as orthophosphate. The method is based on the chemistry of 12-tungstophosphoric acid, H3PW12O40 (TPA). In the procedure, orthophosphate is converted quantitatively to TPA in aqueous solution at a tungstate–phosphate molar ratio ≥24, in the pH range 1.2 to 1.9 and in the temperature range 60 to 100 °C. The TPA is subsequently extracted into n-amyl alcohol and its absorbance measured at 267 nm. Beer's law is followed up to at least 25 µg/ml P. The method is less prone to solution conditions and species interference (especially As(V) and Si(IV)) than the widely used molybdenum-blue methods and is competitive in terms of sensitivity, precision, and accuracy. The TPA method was applied successfully to the determination of phosphate in a variety of phosphorus-containing specimens including natural waters, ores, and alloys.Based on potentiometric and spectrophotometric studies, a scheme which accounts for the observed heptaproticity of TPA is proposed.