The decompositions of peroxyacetic acid, peroxymonosulphuric acid (Caro's acid), and mono-peroxyphthalic acid have been studied in aqueous solution at pH values close to the pK_a of the acids. When precautions are taken to eliminate metal-ion catalysis, essentially quantitative yields of ¹O₂, are obtained, as determined using the caesium salt of anthracene-9,10-bis(ethanesulphonate)(aes) as a trap for ¹O₂. With peroxypivalic acid the maximum yield of ¹O₂, was only ca. 80%, but in this case non-¹O₂ attack on the aes also occurred. Negligible amounts of ¹O₂ are formed when H₂O₂ reacts with monoperoxyphthalate or peroxyacetate anions at pH ca. 10.7 under normal ‘clean’ conditions. However, the presence of the chelating agent diethylenetriamine-NNN′N″N″-penta-(methylphosphonic acid) produces a dramatic decrease in the reaction rates. It is concluded that the reactions of H₂O₂ with peroxy-acid anions are extremely sensitive to catalysis, probably by transition-metal ions.