Two substituted phosphonium tetrahydoxidohexaoxidopentaborate(1-) salts, [ iPrPPh 3][B 5O 6(OH) 4]·3.5H 2O ( 1) and [MePPh 3][B 5O 6(OH) 4]·B(OH) 3·0.5H 2O ( 2), were prepared by templated self-assembly processes with good yields by crystallization from basic methanolic aqueous solutions primed with B(OH) 3 and the appropriate phosphonium cation. Salts 1 and 2 were characterized by spectroscopic (NMR and IR) and thermal (TGA/DSC) analysis. Salts 1 and 2 were thermally decomposed in air at 800 °C to glassy solids via the anhydrous phosphonium polyborates that are formed at lower temperatures (<300 °C). BET analysis of the anhydrous and pyrolysed materials indicated they were non-porous with surface areas of 0.2-2.75 m 2/g. Rhe recrystallization of 1 and 2 from aqueous solution afforded crystals suitable for single-crystal XRD analyses. The structure of 1 comprises alternating cationic/anionic layers with the H 2O/pentaborate(1-) planes held together by H-bonds. The cationic planes have offset face-to-face ( off) and vertex-to-face ( vf) aromatic ring interactions with the iPr groups oriented towards the pentaborate(1-)/H 2O layers. The anionic lattice in 2 is expanded by the inclusion of B(OH) 3 molecules to accommodate the large cations; this results in the formation of a stacked pentaborate(1-)/B(OH) 3 structure with channels occupied by the cations. The cations within the channels have vf, ef (edge-to-face), and off phenyl embraces. Both H-bonding and phenyl embrace interactions are important in stabilizing these two solid-state structures.