The Ni and new Cd and Co complexes of sulfadimethoxine (SDMX) are prepared and investigated their structural changes and stability by heating at different temperatures. The investigations were earned out by means of the infrared spectroscopy (IR) after heating the samples at several temperatures in the range 50-300 °C for an hour. The IR spectra of the heated samples at different temperatures have been compared with those at room temperature. The changes in the fundamental vibrational bands in their IR spectra gave the temperature values at which the samples decomposited. As a result, it is found that the stability of the complexes are higher than SDMX and their stability is in the order Ni > Co> Cd which depends on the second ionization potential of the metals.