Compounds having improved properties of inhibiting the precipitation of metal ions when used in threshold amounts have been made which are derivatives of ammonia or di- or polyamines in which the amine hydrogens have been substituted with both methylenephosphonic acid groups or their salts and hydroxypropyl quaternaryammonium halide groups. Thus, ammonia can have two hydrogens replaced with methylenephosphonic groups while the third is replaced with a hydroxypropyltrimethylammonium chloride group; and three hydrogens of ethylenediamine can be replaced with methylenephosphonic groups, the fourth being replaced with a hydroxypropyltrimethylammonium chloride group.