Oxygen/sulfur-bridged incomplete cubane-type molybdenum aqua clusters [Mo-3(mu(3)-S)(mu-X)(mu-S)(2)(H2O)(9)](4)(+) (X = O, S) in hydrochloric acid react with dien (diethylenetriamine) to give [Mo-3(mu(3)-S)(mu-X)(mu-S)(2)(dien(-))(dien)(2)]Cl-3. nH(2)O [1, X = O, n = 3; 2, X = S, n = 4; dien(-) = NH2(CH2)(2) NH(CH2)(2)NH-], respectively, where each cluster has a deprotonated dien. The X-ray structural analysis of 1 revealed proton dissociation from an amino group of one of the three dien ligands: one Mo-N distance [1.987(4) A] is clearly shorter than the other eight Mo-N distances [2.229(3)-2.276(3) A]. The H-1 NMR spectra of the Mo-dien clusters 1 and 2 in D2O show two well-resolved methylene proton signals in the 2.8- to 3.0-ppm region, which indicates that both deprotonated amines in 1 and 2 receive D+ ions from solvent D2O. The factors for the proton dissociation are discussed.