Abstract: The instant invention is directed to a process for determining the amount of organic phosphonate present in an aqueous solution, comprising the steps of:(a) adding 1000 to 20,000 ppm of a strong oxidizing agent and 8 ppb to 5000 ppm silver ion to the aqueous solution;(b) allowing the organic phosphonate present in said solution to react to form orthophosphate;(c) precipitating any excess silver in said solution by the addition of 1000 to 50,000 ppm of a reducing agent or at least an equimolar amount of a halide, based on the silver ion added in Step (a), and removing any precipitate formed;(d) adding to said solution 5 to 30 percent, by volume, of an acid solution containing 500 to 2000 ppm of ions selected from the group consisting of molybdate ion and vanadate ion, and if a halide was used in Step (c), adding 1000 to 50,000 ppm of a reducing agent;(e) measuring the absorbance of said solution at 625 to 880 nanometers and determining the orthophosphate concentration from a calibration curve of orthophosphate