A method for the treatment of an aqueous system, typically hard water in a cooling system, is described which comprises adding thereto; (i) a phosphonocarboxylic acid which contains at least 3 acid groups which are carboxylic and phosphonic acid groups, such that at least one acid group is a carboxylic acid group and at least one acid group is a phosphonic acid group, at least the said 3 acid groups being attached to carbon atoms, or a water-soluble salt thereof, (ii) an amino phosphonic acid having the formula: H2 - PO3(CH2)n - N (R1)(R2) in which R1 and R2 independently represent hydrogen, alkyl, especially of 1 to 4 carbon atoms, - (CH2)nPO3H2, or -(CH2)nCOOH, and n is 1 or 2, or a water-soluble salt thereof, and (iii) an aminophosphonic acid oxide of the formula: H2 - PO3(CH2)n - N(--O)(R1)(R2) in which R1, R2 and n are as defined above, or a water-soluble salt thereof. a
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