Abstract: A method of treating an aqueous system to inhibit the corrosion of metals in contact therewith and/or inhibit the deposition of scale therefrom comprising adding to the aqueous system a minor proportion of a compound of the formula: ##STR1## wherein n is 0 or 1 and when n is O, R.sub.3 and R.sub.4 are the same or different and each is hydrogen, an alkyl group having from 1 to 4 carbon atoms, a benzyl, allyl, --CH.sub.2 PO.sub.3 H.sub.2 -- or --CHCOOH.CH.sub.2 COOH group or a group of formula ##STR2## wherein R.sub.5 is hydrogen, a methyl, --CH.sub.2 COOH or --CH.sub.2 CH.sub.2 COOH group and Z is a group --COOH or --PO.sub.3 H.sub.2, with the proviso that when Z is a PO.sub.3 H.sub.2 group then R.sub.5 is hydrogen or methyl, and when n is 1, R.sub.1 and R.sub.2 are the same or different and each is hydrogen, or a methyl or phenyl residue and R.sub.3 is hydrogen and R.sub.4 is hydrogen, an alkyl group having from 1 to 4 carbon atoms or a --CH.sub.2 CH.sub.2 COOH group, or R.sub.3 and R.sub.