An apparatus is described for the extraction of ions from flames burning at atmospheric pressure, and for the subsequent mass analysis of the ions. It consists essentially of a fine leak in a platinum or quartz diaphragm on which the flame is played, and on the other side of which a low pressure is maintained by pumping. The ions are separated by suitable disposition of electrodes in a series of expansion chambers before passing into the analyzer. It is concluded that secondary ionization is not important in the mass spectrometer, and that charge exchange reactions and other related types of reaction, in so far as they occur inside the spectrometer, largely reflect similar reactions occurring in the external flame. Ionic concentrations as low as 10
5
per cm
3
can be measured for ions ranging in mass from 10 to 400 atomic units. A brief account is given of the ionization observed from premixed flames of hydrogen, oxygen and nitrogen. The most evident positive ion was hydroxonium (H
3
O
+
), the other important ones being NH
+
4
and NO
+
. The first two of these also occurred in hydrated forms, associated with up to 4 molecules of water. These hydrates occur to a large extent in the cooler parts of the flame system (just before the reaction zone), and are considered to be a secondary phenomenon, possibly formed by association just inside the entry leak into the spectrometer. Experimental evidence is adduced for the formation of H
3
O
+
in the homogeneous gas phase in and near the reaction zone, rather than by catalytic interaction with the walls of the leak. The most likely reaction is considered to be H + H + OH = H
3
O
+
+
e
-
and the kinetics of this are considered. Doubts about the heat of formation of this ion, and about that of the NH
+
4
ion preclude quantitative decisions on many points. The ionization of nitric oxide is shown to be essentially a thermal phenomenon, by observations of ionization with known amounts of nitric oxide added to the flame gases. The relatively slow rate of recombination observed beyond the reaction zone is discussed, and found to be in line with previous results.
