The ions present in a variety of flames have been studied by continuously pumping a small fraction of the hot gases through a small hole and expanding it supersonically inside a conical duct to the low pressures required for mass spectrometric analysis. We conclude from measurements of ion abundances that if a halogen X is added to a flame containing free electrons, then the negative ion X - is produced by dissociative attachment in the forward step of HX + e
-
⇄X
-
+H. Reaction (I) is found to be rapid enough to be equilibrated in the burnt gases. This method of sampling imposes a rapid drop in temperature and pressure on the gas as it expands and typically there is a fall of roughly 1400 K and 99 kPa in the first 10
-7
s. An equilibrium such as (I) relaxes to some extent to these falling temperatures, but at a distance of around two orifice diameters inside the expansion no further shift of (1) is possible. A comparison of a measured [X
-
] with one computed on the basis of (I) being equilibrated in the flame gives a quantitative measure of the extent to which the reaction shifts during sampling. In addition, the flame sample is often also cooled as it passes through boundary layers immediately before entering the instrument. We conclude however that this effect is of little consequence, provided a large enough sampling hole (diameter > 0.15 mm) is used. In this case the measured shift of (I) can be compared with values of it predicted on the basis of guessed velocity constants for the backward process in (I) and also a one-dimensional adiabatic treatment of the expansion. This comparison provides values for the rate constants and of both steps in (I). The magnitudes of k
-1
for the back reaction over the temperature range 1800—2650 K are 7 x 10
-10
, 8 x 10
-10
and 10 x 10
-10
ml molecule
-1
s
-1
for chlorine, bromine and iodine, respectively (with uncertainties corresponding to factors of 1.6, 1.8 and 2.0) and accordingly independent of temperature. The forward dissociative attachment of electrons has
k
1
such that its activation energy is the exothermicity of the reaction. The cross-section (nor2) for this direction is large and the same for each halogen, being 1.5 x 10
-18
m
2
. From the reaction X+ e
-
+ M->X
-
+ M not apparently occurring (M being any molecule acting as a chaperon) in flames, we conclude that its rate coefficient is less than 3 x 10
-29
ml
2
molecule
-2
s
-1
for these three halogens at temperatures of around 2000 K.
