Abstract Radiofrequency spectroscopy has been extended to positive ions with S ground states by means of optical pumping. The ions are stored in buffer gases or ion traps and are polarized directly by optical pumping or indirectly by spin exchange, change exchange, or Penning ionization with optically pumped atoms. The applied methods are described. The experiments can be divided into two categories: Collisional interactions of the polarized ions are investigated, like the exchange processes mentioned above, spin exchange with free electrons, spin depolarization and hyperfine density shifts in rare gases. For the two latter effects drastic differences are observed between atomic and ionic 2S 1 2 states the explanations of which reveal the influence of the ionic charge on the interactions. The comparison of equivalent processes involving isoelectronic 2S or 2P configurations of atoms and ions provides a test of current collision theories. Precision rf spectroscopy of ionic ground states yields the 2S 1 2 hyperfine structure splittings of 3He + and group II ions with relative accuracies of up to 10 −9, with further improvement possible. Direct and indirect optical pumping of 1S 0 ions is used to determine nuclear magnetic moments, diamagnetic shielding coefficients, and chemical shifts. Applications and further developments of the present methods are discussed.