The thermal decomposition of 3 K₂O·7 MoO₃·4 HO (I), K₂O·3MoO₃·3 H₂O (II), K₂O·4 MoO₃·3 H₂O (III) and K₂O·8 MoO₃·12 H₂O (IV), has been studied by means of DTA and TGA curves and IR spectra. The (I) and (II) lose the crystallization water in one stage, the (III) in two stages (2,5 + 0,5 H₂O) and the (IV) in 5 stages (10.5 H₂O below 140° and 1,5 H₂O between 140° and 200°). The activation energies for the various dehydrations have been calculated. The last 1,5 H₂O for the (IV) and the 3 H₂O for the (III) are very important in the hydrated salt structure, because the irreversible structural alters with the elimination of this type of water. After the dehydration only the (II), and the (III) melt unaltered at 567° and 559° respectively; the (I) decomposes in di and trimolybdate and the (IV) in tetramolybdate and MoO₃.