The dehydration of TlB 5 O 6 (OH) 4 • 2H 2 O leads to a loss of water in one step, forming anhydrous TlB 5 O 8 , which is amorphous to X-ray diffraction. This can be explained by structural considerations; a decrease in oxygenated coordination of thallium ions occurs during the heating. TlB 5 O 8 crystallizes at about 450°°C and melts at 660°°C. By slow cooling from the melt, single crystals were obtained. TlB 5 O 8 is orthorhombic, space group Pbca, with unit cell parameters a ؍ 7.557(3) A s , b ؍ 11.925(6) A s , c ؍ 14.734(19) A s ; Z ؍ 8. The structure was solved from 1113 reflections until R ؍ 0.023 and R w ؍ 0.026. The structure consists of two separate interlocking three-dimensional networks of pentaborate groups, which form large empty tunnels. Nine oxygen atoms belonging to four different pentaborate groups surround each thallium ion. Except for the presence of pentaborate groups with almost the same geometry, there is no relation between the structure of TlB 5 O 8 and that of its hydrated precursor, TlB 5 O 6 (OH) 4 • 2H 2 O; the shorthand notation of polyborate anions in these compounds is 5 :R 3 (4 ؉T) and 5 : 4 ؉T, respectively.