The structure of Tl(2)Mo(9)Se(11), dithallium nona-molybdenum undeca-selenide, is isotypic with Tl(2)Mo(9)S(11) [Potel et al. (1980 ▶). Acta Cryst. B36, 1319-1322]. The structural set-up is characterized by a mixture of Mo(6)Se(i) (8)Se(a) (6) and Mo(12)Se(i) (14)Se(a) (6) cluster units in a 1:1 ratio. Both components are inter-connected through inter-unit Mo-Se bonds. The cluster units are centered at Wyckoff positions 3a and 3b (point-group symmetry .). The two Tl(I) atoms are situated in the voids of the three-dimensional arrangement. Two of the five independent Se atoms and the Tl atoms lie on sites with 3. symmetry (Wyckoff site 6c).