Two new uranium(IV) thiosilicates, Cs2Na4 [U-2(SiS4)(2)(Si2S8)] and Cs2.12Na3.88[U-2(SiS4)(2)(Si2S7)], were obtained by flux crystal growth using SiS2 as a silicon source. The former compound contains a novel Si2S86 unit that features a terminal persulfide group. The magnetic susceptibility measurements performed on this compound revealed paramagnetic behavior with a moment of 3.49 mu(B) per uranium atom as obtained from a Curie-Weiss law fit and showed no magnetic transition down to 2 K. The structures are based on closely related isomeric planar and 3D topologies that can be transformed into one another by a rotation of the structural units.