The mixed-ligand saccharin (Hsac) complexes of Co(II), Ni(II) and Cu(II) with N,N'-dimethylethylenediamine (dmen), and N,N-dimethylethylenediamine (ndmen) (Figure 1) were synthesized and characterized by elemental analysis, magnetic susceptibility, spectral (UV-Vis and FTIR) methods, and simultaneous TG, DTG and DTA techniques. The complexes have pseudooctahedral geometries with two dimethylethylenediamine molecules coordinated to the metal ions as chelating ligands through their two nitrogen atoms and two monodentate saccharinato ligands in the trans positions for the complexes of the type [M(sacO)(2)(dmen)(2)] (M = CO(II), Ni(II)), and [M(sac-N)(2)(ndmen)(2)] (M = Co(II), Ni(II), Cu(II)) and two aqua ligands in the trans positions in [Cu(H2O)(2)(dmen)(2)](sac)(2). (sac-O; sac-N = the saccharinato ligand may be coordinated to the metal ions through their carbonyl oxygen or their nitrogen atom, respectively). The decomposition mechanism and thermal stability of the solid complexes are interpreted in terms of their structures. The thermal stability order of the investigated complexes is Cull) > Co(II) > Ni(II) while the bis(N,N',-dimethylethylenediamine) complexes are thermally less stable than those of bis(N,N-dimethylethylenediamine). The final decomposition products-the respective metal oxides-were identified by FT-IR spectroscopy.