The imidazole ring and its derivatives are very useful models to understand the coordination properties and the reaction mechanisms of the biologically important systems where these molecules are involved. Six different complexes of imidazole-4-acetic acid (IAA) with Co(II), Ni(II) and Cu(II) were precipitated and characterized by elemental analysis, UV-VIS and infrared (IR) spectroscopy. The thermal stability was determined by differential scanning calorimetry (DSC) and by thermogravimetric analysis (TGA), and the decomposition mechanisms was investigated by coupling on-line thermogravimetry and IR spectroscopy to obtain the evolved gas analysis (EGA by TG-FTIR).