Reversibly self-assembling host-guest complexes of the softball type are characterized by electrospray ionization mass spectrometry. Quaternary ammonium ions serve simultaneously as guests and ion labels. Isotope pattern analysis, inclusion of labeled guests, heterodimer experiments, size and shape dependence, and collision-induced fragmentation reveal that the capsular structure of these hydrogen-bonded complexes is retained in the gas phase with the guests inside their cavity. The results parallel findings from solution-phase NMR experiments and show that the guests are encapsulated inside the cavity. These results also demonstrate the value of mass spectrometry for high-order structural examination of supramolecular architecture.