Halogens, when added to flames, usually form negative ions. In addition, iodine produces I⁺, I⁺₂, IO⁺ and I²⁺, whereas bromine gives Br⁺ and Br⁺₂, but chlorine forms no positive ions whatsoever. The dominant of these species are the monatomic ions I⁺ and Br⁺, which are evidently produced in the reaction zone of the flame, partly by charge transfer from primary ions, such as CHO⁺, which are generated during the combustion of a hydrocarbon. Otherwise, the sequences I + O → IO⁺+ e^–, IO⁺+ H → I⁺+ OH and, to a lesser extent, I + I → I⁺₂+ e^–, I⁺₂+ H → I⁺+ HI can be important. Only the origin of I²⁺ remains a puzzle.

The disappearance of I⁺ from the burnt gases of flames from 1820 to 2650 K has been studied in detail and proves to be by three routes involving: I⁺+ e^–+ M → I + M, (III), I⁺+ I^–→ I + I, (IV), I⁺+ H₂O = I⁺ . H₂O and (– XV), I⁺ . H₂O + H → H₃O⁺+ I. (– XIV)

The three-body recombination coefficient of reaction (III) is large, being 1.6 ± 0.8 × 10^–23T^–1 cm⁶ molecule^–2 S^–1, in line with other measurements for metal ions recombining with electrons in flames. The two-body ion–ion recombination step (IV) is, however, surprisingly rapid, having k₄= 4.9 ± 2.0 × 10^–7T^–1/2 cm³ ion^–1 S^–1. The rate coefficient for the last of these steps is also large, being k_–14= 1.1 × 10^–8 with iodine and 5.9 × 10^–9 cm³ molecule^–1 S^–1 for the analogous reaction with Br, both being correct to within a factor of 3. Finally, we conclude that the first hydration energies of I⁺ and Br⁺ are, respectively, 25 ± 12 and 45 ± 20 kJ mol^–1.