Observations are reported on the change in electron concentration in the burnt gases of hydrogen + oxygen+nitrogen flames containing small amounts of acetylene and controlled traces of alkali elements when amounts of halogens up to ca. 1 % are added. In many cases the electron concentration passes through a pronounced maximum. The results are interpreted in terms of a mechanism advanced previously,1 the essential point of which is that whereas alkali atoms A are ionized by collisional processes with third bodies X in the absence of halogen, A+X⇌A++e–+X, the presence of halogen allows an electron exchange mechanism A+B⇌A++B– to operate, B representing an atom of halogen. Rate constants are deduced for the reaction of A+ with B–, and compare reasonably with theoretical estimates.