Two new organic-inorganic hybrid compounds of molybdenum(VI) (HDBU) 3 (NH 4 )[b-Mo 8 O 26 ] Á H 2 O 1 and (HDBU) 4 [d-Mo 8 O 26 ] 2 have been synthesized and characterized by elemental analyses, FT-IR and UV-vis spectroscopies, and single-crystal X-ray diffraction (at 293 and 100 K, respectively). Compound 1 is obtained at room temperature by adding 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) on aqueous molybdate solution at pH ¼ 5.7. Compound 2 is obtained hydrothermally from 1 (110 1C, 24 h, autogenous pressure). Compound 1 crystallizes in the triclinic crystal system, space group P 1, with a ¼ 11.2492( 9), b ¼ 14.1907(10), c ¼ 16.7498(10) A ˚, a ¼ 80.993(4), b ¼ 72.272(6), g ¼ 71.696(7)1, V ¼ 2412.5(3) A ˚3 at T ¼ 20 1C and Z ¼ 2. The refinement of the structure leads to a residual factor R ¼ 0.0697 for 10234 independent observed reflections [I/s(I)X2] and 623 parameters. Compound 2 crystallizes in the triclinic crystal system, space group P 1, with a ¼ 11.3158(9), b ¼ 11.3773(8), c ¼ 13.2884(17) A ˚, a ¼ 92.110(8), b ¼ 112.127(9), g ¼ 117.581(7)1, V ¼ 1357.4(3) A ˚3 at T ¼ À173 1C and Z ¼ 2. The refinement of the structure leads to a residual factor R ¼ 0.0226 for 11812 independent observed reflections [I/s(I)X2] and 354 parameters. The structure of 1 consists of b-[Mo 8 O 26 ] 4À anions with HDBU + and NH 4 + cations connected to the mineral parts by hydrogen bonds and the structure of 2 contains d-[Mo 8 O 26 ] 4À anions with HDBU + counter-cations. Both the thermal and chemical isomerizations of the two [Mo 8 O 26 ] 4À isomers are highlighted and discussed. Photochromic behaviors of compounds 1 and 2 are also reported.