Evidence is presented for cyclization to yield 2bora-l,3-diazacycloalkanium cations in the gas phase. While the neutral compounds in solution and solid phase are known to possess an acyclic structure (as revealed by X-ray diffraction), the gaseous cations (from which borohydride BH4- ion has been expelled) have a cyclic structure, as revealed by InfraRed Multiple Photon Dissociation (IRMPD) spectroscopy and collisionally activated decomposition (CAD). The IRMPD decomposition of the monocyclic ions proceeds principally via H-2 expulsion, although CAD experiments show additional pathways. Pyrolyses of solid monomeric salts and small oligomers produce higher polymers that are consistent with H-2 expulsion as the major pathway. Deuterium labeling experiments show that scrambling occurs prior to IRMPD or CAD decomposition in the gas phase.