The invention concerns the use, to remove magnetite deposits in water-supply systems, of a combination of (a) at least one phosphonic acid; (b) at least one oxidizing agent; and (c) at least one short-chain organic carboxylic acid selected from the group comprising ascorbic acid, tartaric acid, formic acid, oxalic acid and citric acid; in acid aqueous solutions with a pH between 0 and 3. The total amount of (a) plus (b) is 1 to 10 % by wt., relative to the aqueous solution, and the ratio by weight of (a) to (b) is in the region of 5:1 to 1:10 while the ratio by weight of (a) plus (b) to (c) is in the region of 5:1 to 1:5.