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: wherein Z=—CHR1PO3R2 R=H, CH3, C2H5 or M R1=H, CH3, CR3, C6H5, SO3H2 M=alkali metal or ammonium ion n=2-6, preferably 2-4 m=2-6, preferably 2-4 a=2-10, preferably 2-4 b=2-10, preferably 2-6 x=0-6, preferably 0-3 y=0-6, preferably 0-2 provided that when y=o at least one of n or m is greater than 2.