A series of sulfur/oxygen-bridged incomplete cubane-type molybdenum cluster compounds [Mo3OnS4-n(DTP)(4)(L)] (n = 0-3; DTP = diethyl dithiophosphate, L = monodentate ligand), and their ligand substitution derivatives were prepared by the reaction of the corresponding aqua ions with the ligand DTP. The crystallographic results obtained are as follows. [Mo3OS3(DTP)(4)(CH3CN)] : triclinic, P ($) over bar 1, a = 14.134(13) Angstrom, b = 16.413(16) Angstrom, c = 0.654(9) Angstrom, alpha = 98.08(10)degrees, beta = 112.23(6)degrees, gamma = 77.03(9)degrees, V = 2225.3(37) Angstrom(3), Z = 2, no. of unique data (F-0 greater than or equal to 6 sigmaF-0) = 4810, R = 4.79%. [Mo3OS3(OAc)(DTP)(2)(Py)(2)(OC2H5)] : triclinic, P ($) over bar 1, a = 13.985(7) Angstrom, b = 14.359(7) Angstrom, c = 12.012(4) Angstrom, alpha = 96.01(3)degrees, beta = 104.37(3)degrees, gamma = 114.20(3)degrees, V = 2072.3(17) Angstrom(3), Z = 2, no. of unique data (F-0 greater than or equal to 3 sigmaF-0) = 4944, R = 4.62%. Of four DTP ligands in [Mo3OS3(DTP)(4)(CH3CN)], three are terminal and one is bridging. Both DTP ligands in [Mo3OS3(OAc)(DTP)(2)(Py)(2)(OC2H5)] are terminal, and an acetato ligand is bridging. P-31 NMR spectra of these cluster compounds were measured in a solution of o-dichlorobenzene and acetonitrile. The P-31 NMR spectra of these cluster compounds presented much information about the structure, interaction between the ligands and the cluster core, solvent effect, lability of the ligands, and isomerization of this duster compound series. The assignment of the chemical shifts to each distinct ligand under different circumstances was successfully accomplished. The site of the loosely-coordinated solvent ligand L was kinetically active and informative to the assignments of the spectra. The symmetry and chemical properties of the central trinuclear cluster core Mo3OnS4-n (n = 0-3) were also well reflected in their spectral appearance.