Publisher Summary This chapter discusses electron-tunneling reactions between inorganic ion radicals and transition-metal compounds. Radiolysis of the vitreous aqueous solutions of H2SO4 and NaOH is known to yield the anion radicals SO4- and O- that possess strong electron-acceptor properties and are readily recorded by the characteristic electron paramagnetic resonance spectra. The tunneling reactions of organic ion radicals are discussed. Electron tunneling between organic species was first detected, by direct kinetic experiments, for the reactions of the biphenyl anion radical with naphthalene and pyrene and triphenylethylene. Electron-tunneling reactions between metal compounds are also given. Electron excitation of a molecule results in a decrease of its ionization potential. Electron tunneling from electron-excited donor molecules to acceptors may display itself, for example, in quenching the luminescence of excited molecules. Tunneling recombination of donor–acceptor pairs in crystals is briefly described and a comparison of the model of direct electron tunneling with alternative model is given.