Decomposition of N-(2-hydroxyethyl)ethylenediam inetriacetate (HEDTA) in alkaline solutions containing H2O2, S2O 8 2− , and ClO− and under the action of ozone was studied titrimetrically. HEDTA decomposes in the presence of cobalt or copper salts at stepwise addition of H2O2. In the reaction between HEDTA and persulfate ion, an induction period was observed. It shortens with increasing alkali concentration, or temperature, or with decreasing the initial HEDTA concentration. Nitrite ion does not affect the complexone decomposition. The process is initiated by thermal dissociation of persulfate ions into radical ions followed by chain reaction. Hypochlorite ions effectively decompose HEDTA. Porducts of HEDTA decomposition are oxidized by ozone and persulfate mainly to oxalate and carbonate.