An ion beam technique has been developed that combines some of the methods of mass spectrometry and molecular spectroscopy and is designed for the study of molecular ions at energy levels lying very close to the lowest dissociation limit. Microwave radiation is used to drive spectroscopic transitions, and electric field dissociation of the weakly bound levels provides a high degree of state selection for sensitive detection of the spectra. The analysis of the spectra requires unconventional approaches to the description of the long-range levels and their spectroscopic study and provides stringent tests of ab initio theories.