The ion chemistry of the E and F layers depends on a few ion-neutral reactions, with NO+as the terminating ion, and on dissociative recombination. In the topside ionosphere the main source and sink of the H+ions is the reversible charge transfer process H+ O+⇆ H++ O; the He+ions are formed by the action of solar radiation and are neutralized in dissociative charge transfer collisions with nitrogen molecules. The metallic positive ions present in sporadic E and in the D region result from photoionization and charge transfer. It is not certain how they recombine. Positive ions in the middle and lower part of the D region take part in a sequence leeding to the hydrated protons H3O+(H2O)nobserved. Negative ions are here important but little can be said with asurance about their nature.