The chemical structure of a premixed fuel-rich methane flame has been investigated by using a four-stage modulated molecular beam inlet to a quadrupole mass spectrometer. The premixed, laminar, flat flame analysed had the following composition (all molar percentages) and conditions: 26.7% CH
4
- 33.3% O
2
- 40.0% Ar (
ϕ
= 1.60), pressure = 4.13 kPa, cold-gas velocity at 293 K is 0.80 m s
-1
. Mole fraction profiles throughout the flame were measured for the stable species CH
4
, O
2
, H
2
O, H
2
, CO, CO
2
, C
2
H
2
, C
2
H
4
and for the radical species H, OH, CH, CH
2
, CH
3
. These profiles were used to determine the major reaction routes of the CH
i
(
i
= 3, 2, 1) radicals within the fuel-rich flame. It is concluded that in rich flames the methane is consumed mainly through CH
4
+ H → >CH
3
+ H
2
. (2) The methyl radical then undergoes either recombination to form C
2
-hydrocarbons or hydrogen abstraction via CH
3
+ H → CH
2
+ H
2
. (12) This is a significant route for the consumption of the methyl radical throughout the flame and is the major route for CH
2
production. The majority of the CH
2
is then consumed via CH
2
+ O
2
→ > products. (16) This is an important oxidation route in the rich methane flame. The CH radical is produced and consumed in reactions analogous to (12) and (16), but this is a relatively minor reaction path. A rate constant of 2.2 x 10
13
cm
3
mol
-1
s
-1
at 1810 K is calculated for reaction (16) by using a rate constant for (12) of 1.0 x 10
13
cm
3
mol
-1
s
-1
at 1770 K. This value of
k
12
is consistent with the results of the present work and falls within the range of values calculated in the paper. Molecular-beam inlet mass spectrometry may be used to obtain accurately spatially resolved concentration profiles through flames of stable and radical species. The main limitation of the technique is the wide error limits on the measured radical concentrations.