Covers the colorimetric determination of phosphonate in the range of 0.1 to 5 mg/L in brines from gas and oil production operations, water injection/disposal operations, and other industrial applications. This test method is intended for use in analyzing low concentrations of phosphonate in brine containing interfering elements. It includes a digestion step to break down the phosphonate materials to orthophosphate, and has been used successfully with reagent water and both field and synthetic brines. Keywords: brine, corrosion testing, phosponate.