Abstract: A method for inhibiting the formation of scale in waters having a high barium content and low pH comprises the addition of an aqueous solution of a phosphonate of formula: ##STR1## wherein Z=--CHR.sup.1 PO.sub.3 R.sub.2R=H, CH.sub.3, C.sub.2 H.sub.5 or MR.sup.1 =H, CH.sub.3, CR.sub.3, C.sub.6 H.sub.5, SO.sub.3 H.sub.2M=alkali metal or ammonium ionn=2-6, preferably 2-4m=2-6, preferably 2-4a=2-10, preferably 2-4b=2-10, preferably 2-6x=0-6, preferably 0-3y=0-6, preferably 0-2provided that when y=0 at least one of n or m is greater than 2.The phosphonates may be present in the form of a fully or partially neutralised salt and may be injected into production fluids obtained from an underground reservoir or injected into the reservoir via a wellbore as part of a squeeze technique.