[Ph 4 P][B 5 O 6 (OH) 4 ]·1.5H 2 O (1) was obtained as colourless crystals in high yield from a MeOH/H 2 O solution of B(OH) 3 and [PPh 4 ][OH] (5:1). A single-crystal X-ray study confirmed that the solid-state structure was comprised of supramolecular networks of interconnected anions (H-bonds) and cations (phenyl embraces). Crystals are triclinic, P1¯ with a=13.3174(3)Å, b=13.3180(3)Å, c=16.2055(4)Å, α=91.1950(10)°, β=91.180(2)°, γ=105.456(2)°, T=120(2)K, V=2768.66(11)Å 3 and Z=4. TGA/DSC analysis showed that 1 thermally decomposed in air at 800°C to 2.5B 2 O 3 , via a low temperature dehydration step to a condensed pentaborate, [PPh 4 ][B 5 O 8 ].