Reaction between two oxides leading to the formation of a new compound through a change in weight can be studied by TG methods. However, for catalyst preparations, in most of the cases the starting materials are not oxides. The more common methods of coprecipitation or kneading of the precipitated oxides are employed. In such cases the TG and DTG curve of the composites are of a complex nature for several possible reasons including the formation of new compounds. The interpretation of the conventional DTG curve of such a sample and identification of temperature regions for any new thermal activity thus presents problems. For such studies the use of a derived derivative thermogravimetric curve is suggested as described in this paper. This curve indicates the differences between the experimental thermal behaviour of the composite vis-a-vis theoretical thermal behaviour of the composite computed from the thermal behaviour of its constituents. The regions of a new thermal activity can be readily located and interpretated. Two systems are described, MgO-Cr2O3 and MgO-Fe2O3.