1. The saturation current is a means of directly determining the activation energy of ion formation in flames. A knowledge of the activation energy of ion formation is necessary to determine the reactions leading to the primary formation of ions in flames. 2. We have measured the activation energies of the process of ion formation in the reaction and burnt gas zones of acetylene-air and acetylene-oxygen flames. The ion formation mechanism in these flames is the same, since the numerical values of E for the corresponding zones of these flames differ only slightly, in spite of the appreciable difference in flame temperatures. 3. Transition from the reaction zone to the burnt gas zone is accompanied by a sharp change in the activation energy of ion formation. This indicates that the mechanisms of ion formation in these parts of the flame are different. 4. Seeding with alkali metals (Na, K) reduces the activation energy in the burnt gas zone of an acetyleneair flame, obviously owing to the appearance of a new ion formation mechanism different from that observed in the same zone in the absence of alkali metals. 5. The fact that the value of the activation energy of formation of the excited CH radical is close to the value of the activation energy of ion formation in the reaction zone of an acetylene-oxygen flame indicates a possible connection between the ionization process and the process of CH radical excitation. The excited CH radical is the most likely source of ion formation in the flame reaction zone.