The synthesis and characterization of six new substituted guanidium tetrahydroxidohexaoxidopentaborate(1-) salts are reported: [C(NH 2) 2(NHMe)][B 5O 6(OH) 4]·H 2O ( 1), [C(NH 2) 2(NH{NH 2})][B 5O 6(OH) 4] ( 2), [C(NH 2) 2(NMe 2)][B 5O 6(OH) 4] ( 3), [C(NH 2)(NMe 2) 2][B 5O 6(OH) 4] ( 4), [C(NHMe)(NMe 2) 2][B 5O 6(OH) 4]·B(OH) 3 ( 5), and [TBDH][B 5O 6(OH) 4] ( 6) (TBD = 1,5,7-triazabicyclo [4.4.0]dec-5-ene). Compounds 1- 6 were prepared as crystalline salts from basic aqueous solution via self-assembly processes from B(OH) 3 and the appropriate substituted cation. Compounds 1- 6 were characterized by spectroscopic (NMR and IR) and by single-crystal XRD studies. A thermal (TGA) analysis on compounds 1- 3 and 6 demonstrated that they thermally decomposed via a multistage process to B 2O 3 at >650 °C. The low temperature stage (<250 °C) was endothermic and corresponded to a loss of H 2O. Reactant stoichiometry, solid-state packing, and H-bonding interactions are all important in assembling these structures. An analysis of H-bonding motifs in known unsubstituted guanidinium salts [C(NH 2) 3] 2[B 4O 5(OH) 4]·2H 2O, [C(NH 2) 3][B 5O 6(OH) 4]·H 2O, and [C(NH 2) 3] 3[B 9O 12(OH) 6] and in compounds 1- 6 revealed that two important H-bonding R 2 2(8) motifs competed to stabilize the observed structures. The guanidinium cation formed charge-assisted pincer cation-anion H-bonded rings as a major motif in [C(NH 2) 3] 2[B 4O 5(OH) 4]·2H 2O and [C(NH 2) 3] 3[B 9O 12(OH) 6], whereas the anion-anion ring motif was dominant in [C(NH 2) 3][B 5O 6(OH) 4]·H 2O and in compounds 1- 6. This behaviour was consistent with the stoichiometry of the salt and packing effects also strongly influencing their solid-state structures.