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Two substituted phosphonium tetrahydoxidohexaoxidopentaborate(1-) salts, [ⁱPrPPh₃][B₅O₆(OH)₄]·3.5H₂O (<b>1</b>) and [MePPh₃][B₅O₆(OH)₄]·B(OH)₃·0.5H₂O (<b>2</b>), 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 <b>1</b> and <b>2</b> were characterized by spectroscopic (NMR and IR) and thermal (TGA/DSC) analysis. Salts <b>1</b> and <b>2</b> 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 m²/g. Rhe recrystallization of <b>1</b> and <b>2</b> from aqueous solution afforded crystals suitable for single-crystal XRD analyses. The structure of <b>1</b> comprises alternating cationic/anionic layers with the H₂O/pentaborate(1-) planes held together by H-bonds. The cationic planes have offset face-to-face (<i>off</i>) and vertex-to-face (<i>vf</i>) aromatic ring interactions with the ⁱPr groups oriented towards the pentaborate(1-)/H₂O layers. The anionic lattice in <b>2</b> 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 <i>vf</i>, <i>ef</i> (edge-to-face), and <i>off</i> phenyl embraces. Both H-bonding and phenyl embrace interactions are important in stabilizing these two solid-state structures.