Crystal structures of -CsB 5 O 8 , -CsB 5 O 8 , and -RbB 5 O 8 have been determined from single-crystal X-ray di4raction data. -CsB 5 O 8 crystallizes in the monoclinic space group P2 1 /n with the following parameters: a ؍ 7.117(2) A > , b ؍ 9.634(3) A > , c ؍ 10.391(4) A > , ؍ 101.160(4)3; Z ؍ 4; the structure was re-5ned from 1392 re6ections until R 1 ؍ 0.0489. -CsB 5 O 8 and -RbB 5 O 8 have been found to be isostructural with -KB 5 O 8 and -KB 5 O 8 (and TlB 5 O 8 ), respectively. They crystallize with eight formula units in the Pbca space group. Unit cell parameters are a ؍ 8.697(3) A > , b ؍ 8.431(2) A > , c ؍ 21.410(6) A > for -CsB 5 O 8 and a ؍ 7.553(2) A > , b ؍ 11.857(3) A > , c ؍ 14.813(4) A > for -RbB 5 O 8 ; their structures were re5ned from 1085 re6ections until R 1 ؍ 0.0712, and from 1271 re6ections until R 1 ؍ 0.0290, respectively. The structures of these compounds can be described on the basis of a new two-( -CsB 5 O 8 ) and three-dimensional ( -CsB 5 O 8 , -RbB 5 O 8 ) network of pentaborate units [B 5 O 8 ] ؊ ; these units are formed by two B 3 O 3 cycles linked by a boron atom, which contain two groups of two BO 3 triangles ( ) on each side of a BO 4 tetrahedron (T); their shorthand notations are 5:R 2 (4 ؉T) for -CsB 5 O 8 and 5:R 3 (4 ؉T) for -CsB 5 O 8 and -RbB 5 O 8 . Distortions of the [B 5 O 8 ] ؊ units found in these compounds are presented.