Mass spectrometric measurements have been made of the concentrations of ions present in flames of H2, O2 and N2 with trace quantities of C2H2 added, as well as one of the halogens Cl, Br or I. The principal charged species found were H3O+, e– and X–, where X is a halogen atom. The hydronium ion is produced in the reaction zone of a flame in amounts greater than those for equilibrium at the temperature of the burnt gases. Recombination of charged species thus occurs and proceeds by two reactions; H3O++ X–1→ H2O + HX (I), H3O++ e–1→ H + H + OH. (II) From a study of the rate of decay of H3O+ for varying amounts of one of the halogens present, it proved possible to measure the ratio of the rate coefficients (k1/k2) of reactions (I) and (II). For each halogen k1/k2 was constant over the temperature range 1810–2570 K and was 0.10 ± 0.03 for Cl, 0.07 ± 0.02 for Br and 0.03 ± 0.02 for I. The resulting values of k1 are also independent of temperature and are 4.1 ± 1.6 × 10–8, 2.7 ± 1.0 × 10–8 and 1.2 ± 0.7 × 10–8 ion–1 cm3 s–1 for Cl, Br and I, respectively. The corresponding cross-sections (πσ2) are 2.1, 1.5 and 0.7 × 10–17 m2. The heats of reaction (I) are large and the products appear to be H2O and HX with the latter either electronically excited or completely dissociated, but not in its ground electronic state.