A method of preventing scale from depositing in underground petroleum-containing formations and upon equipment used to inject into or remove from such formations high brine, scale-forming waters. This method comprises treating such waters with a scale-inhibiting amount of an acrylic acid or acrylamide homopolymer or co-polymer of acrylic acid with acrylamide having a molecular weight within the range of 1,000-50,000 which have been modified to contain up to about 30 mole percent of amido(C.sub.2 -C.sub.6 alkyl)phosphinic acid groups and the alkali metal, ammonium and amine salts thereof.