Thiosemicarbazones and their transition metal complexes are interesting compounds due to antibacterial, antiviral and antitumor activities observed for some of them [1]. By the reaction of (NH4)2MoOCl5 and the corresponding thiosemicarbazones in acetonitrile and in argon atmosphere we prepared single crystals of two molybdenum(V) thiosemicarbazonato complexes: [MoOCl2{;SalTSC-4-Ph};] (1 ; SalTSC-4-Ph=salicylaldehyde-4-phenylthiosemicarbazonato) and [MoOCl2{;NaphtTSC-4-Ph};] c CH3CN (2 ; NaphtTSC-4-Ph = naphtylaldehyde-4-phenylthiosemicarbazonato). The by-product of both synthetic reactions was ammonium chloride. In both 1 and 2 the MoV coordination is a distorted octahedron with the chloro ligands occupying mutually trans positions and with the meridionally bonded deprotonated tridentate (N, O, S) thiosemicarbazone ligand. Although crystallised in different space groups (1 in Pbca, 2 in P21/n), in both crystal structures molecules of the complex are connected in infinite chains by forming N-H...Cl hydrogen bonds. Also, in both structures there are similar π ...π interactions between the aromatic moieties. We also prepared crystals of NH4[MoOCl4(CH3CN)](3) from the acetonitrile solution of (NH4)2MoOCl5. This suggests [MoOCl4(CH3CN)]- to be the reaction intermediate in the synthesis of Mo(V) thiosemicarbazonato complexes. In 3 MoV is in distorted octahedral environment with the acetonitrile molecule being trans to the oxo group. It crystallises in the space group Cmcm with both ammonium cation and [MoOCl4(CH3CN)]- lying at the intersection of two mirror planes. The failure of the synthesis of 1 and 2 from the analogous pyridinium salt, (PyH)2[MoOCl5] (Py = pyridine), could be explained by the non-existence of [MoOCl4(CH3CN)]- anions in its acetonitrile solution. Namely, unstable crystals of (PyH)2[MoOCl5]cCH2Cl2 (4) belonging to the space group Pnma were obtained from acetonitrile and dichloromethane (1:1) solution of (PyH)2[MoOCl5]. In 4 two pyridinium cations are hydrogen bonded to the chlorine atom trans to the oxo group in [MoOCl5]2-. [1] Belicchi Ferrari M., Bisceglie F., Pelosi G., Tarasconi P., Albertini R., Dall’ Aglio P. P., Pinelli S., Bergamo A., Sava G., J. Inorg. Biochem., 2004, 98, 301.