[en] Microwave spectroscopy has now been successfully applied to the study of four molecular ions, HCO⁺, HN₂⁺, CO⁺, and HCS⁺, and for the first two of these we have employed the isotope substitution method to obtain complete ''rsub(s)'' type molecular structures, the most accurate structures now available for any polyatomic molecular ions. We are presently developing several improvements in technique that should make possible similar studies of a number of other interesting small ions. (author)