Cyclic (subset of = N+<, subset of = imidazolidine 3 and 4, hexahydro-pyrimidine 7 and 8, tetrahydro-1,3,5-oxadiazine. 12, and triazoline 15 and 16) and acyclic [(R2N)(2)C = N+<, 19] guanidinium-based salts were synthesized via the quaternization of guanidine derivatives with nitric or perchloric acid or with iodomethane followed by metathesis reaction with silver nitrate, silver perchlorate, or ammonium dinitroamide. The structure of 15d was confirmed by single-crystal X-ray analysis. Most of the salts exhibited low melting points and good thermal stabilities. Their densities range between 1.2 and 1.5 g/cm(3). Standard molar enthalpies of formation were calculated from experimentally determined constant-volume combustion energies obtained using an oxygen bomb calorimeter.