Current studies describe the application of simultaneous TG–DTA–FTIR technique in the in-situ synthesisof tri uranium octaoxide (U₃O₈) and impurity characterization of the ammonium diuranate (ADU) matrix.Formation of U₃O₈ could be due to the decomposition of ADU to uranium trioxide (UO₃) and finally to U₃O₈ by the temperature of 500^◦C. Presence of carbon and carbonate could be seen as impurities in thematrix. Carbon content has been determined quantitatively using TG and FTIR techniques and the resultsare in good agreement. Ammonium nitrate was found as impurity in ammonium diuranate matrix. ThePXRD studies confirm the formation of U₃O₈ as end product during the decomposition of ammonium diu-ranate. From our studies, it is suggested that ammonium diuranate decomposes to tri uranium octaoxide(U₃O₈) through evolution of nitrogen, hydrogen and oxygen, and not through ammonia evolution in theliterature.