E.s.r. spectra are reported for the radicals obtained by the oxidation of a variety of organic compounds by the titanium(III)–persulphate redox system. Evidence is obtained that the sulphate radical anion is formed throughout the pH range employed (1–10·5) and that it can react with organic compounds in at least three ways: by abstracting a hydrogen atom from saturated carbon, by adding to unsaturated or aromatic carbon, and by removing one electron from carboxylate anions and from certain neutral molecules. It displays a marked degree of selectivity in its addition reactions and it behaves differently from the hydroxyl radical in several respects.