Abstract: A process for measurement of low levels of phosphonates in brine waters wherein the phosphonate samples are acidified with hydrochloric acid promptly upon removal from an oil or gas well to prevent phosphonate precipitation or adsorption. The hydrochloric acid acidified phosphonate is digested in the presence of an oxidizing agent at elevated temperature and pressure for sufficient time to oxidize substantially all of the phosphonate to orthophosphate. The phosphorous containing compound is extracted with an organic extractant, preferably selected from the group consisting of methyl iso-butyl ketone/cyclohexane mixture and ethyl acetate, followed by color development for absorption spectrophotometry.