Thermal decomposition and hydrogen reduction of ammonium thiomolybdates of formulas: (NH 4 ) 2 [MoS 4 ] (I), (NH 4 ) 2 [Mo 2 (S 2 ) 6 ] 2H 2 O (II) and (NH 4 ) 2 [Mo 3 S(S 2 ) 6 ] 1.85H 2 O (III), were investigated using thermogravimetric analysis (TGA), either in pure N 2 or in 10% H 2 in N 2 , and temperature-programmed reduction (TPR). The thiomolybdate compounds were characterized by several physicochemical techniques, including FTIR, PAS/FTIR, UV-visible spectroscopy, XRD and CHNS elemental analyses. The decompositions of (I) and (II) proceed in a similar way independently of the type of gas phase, with production of MoS 3 . However, the anion cluster of (III) is decomposed at lower temperatures in the presence of H 2 . The amorphous MoS 3 compound is very sensitive to the presence of H 2 , which also lowers the onset of its decomposition. These results seem to support a model for the structure of MoS 3 consisting of triangular Mo 3 cluster units, similar to those present in (III). TPR could be used as a fingerprint technique for identification of the complexes, as they present strong differences in the shapes and characteristic temperatures of the reductograms.