Phosphonates are widely used as scale and corrosion inhibitors in the industrial environment, but methods of measurement especially at low concentration levels are subject to numerous interferences. The process described herein consists of a digestion step to break-down the phosphonate materials to orthophosphate. The orthophosphate material is extracted into a novel organic phase and measured. The process of extraction eliminates the common interferences to the orthophosphate measurement and allows the accurate determination of phosphonate materials at low levels. The method is suited for measuring phosphonate concentration between 0.1 to 10 mg/1 in brine with a reproducibility of ± 5% at normal concentration. In two hours as many as 20 phosphonate samples can be analyzed by one person using this technique.