The hydrothermal reaction of MoO3, Na2Se2 and Me(4)NCl (molar ratio, 1:3:2) at 135 degrees C for 3 days gave dark red needles of (Me(4)N)(2)Mo3Se13 (I); the reaction of MoO3 and K2Se2 (molar ratio of 1:2 for one month and 1:3-5 for 3 days) at 135 degrees C gave black needle-like crystals of K2Mo3Se12.5O0.5 (II) and black chunky crystals of K6Mo6-Se-27. 6H(2)O (III) respectively. The structures were determined by single crystal X-ray diffraction techniques. Compound I crystallizes in the trigonal space group P3(i)m (No. 157), Z = 1, a = 11.578(1) Angstrom, c = 6.110(2) Angstrom, V = 709.4(3) Angstrom(3). d(calc) = 3.42 g/cm(3), mu = 177.99 cm(-1). Number of data measured : 992. Number of unique data : 502. Number of data having I > 3.0 sigma(I) : 438. Number of variables = 49. R/R(w) = 6.0/7.5%. Compound II crystallizes in the monoclinic space group Cm (No. 8), Z = 2, a = 11.874(3) Angstrom, b = 16.834(3) Angstrom, c = 5.820(3) Angstrom, beta = 116.65(2)degrees, V = 1040(1) Angstrom(3), d(calc) = 4.45 g/cm(3). mu = 246.64 cm(-1). Number of data measured : 997. Number of unique data : 951. Number of data having I > 3.0 sigma(I) : 849. Number of variables = 89. R/R(w) = 4.3/5.3%. III crystallizes in the monoclinic space group P2(1)/n (No. 14), Z = 4, a = 10.21(2) Angstrom, b = 17.84(1) Angstrom, c = 27.04(2) Angstrom, beta = 98.8(1)degrees, V = 4868(21) Angstrom(3), d(calc) = 4.15 g/cm(3). mu = 219.98 cm(-1). Number of data measured : 8706. Number of unique data : 7920. Number of data having I > 3.0 sigma(I) : 3520. Number of variables = 391. The [Mo3Se13](2-) in I and [Mo3Se12.5O0.5](2-) in II are isostructural with [Mo3S13](2-), while III is the cocrystallization of two K2Mo3Se13 clusters and one K2Se. The anions of I and II are the same except that the triply bridging Se site is partially occupied by O atoms in II. Solid state IR and UV/vis spectra are reported. The [Mo3Se13](2-) cluster has been found to possess significant affinity for anionic species, and this has been manifested in all currently known compounds containing this trinuclear cluster. The new compounds reported in this paper also exhibit clearly defined anion-anion weakly bonding interactions. Extended Huckel calculations were performed to explain the unusual [Mo3Se13](2-). Se2- interactions in these compounds. The calculations show that the Se atoms (of the bridging Se-2(2-)), which form the points of interaction of the cluster with the anionic species, are weakly positively charged.