The thermal decomposition properties of Mg-Fe hydrotalcites were studied through in situ Fe-57 Mossbauer spectroscopy and in situ X-ray diffraction. Abrupt changes in the quadrupolar splitting measured in the Mossbauer spectra revealed a phase transition from the starting lamellar structure to a new crystalline arrangement. By analyzing the Mossbauer parameters we show that the material is highly disordered in the 300-400 degrees C temperature range. This hypothesis is confirmed by the X-ray results whose diffractograms indicated the collapse of the lamellar structure and the formation of a solid solution.