Simulated inert matrix fuels corresponding to the compositions  Zr_0.75Y_0.10Er_0.05Ce_0.10O_1.925and Zr_0.70Y_0.10Er_0.05Ce_0.15O_1.925were synthe-sized through the ammonia internal gelation process. The thermal decomposition behaviour of the gel was investigated using thermogravime-try/differential thermal analysis (TG/DTA) and powder X-ray diffraction (XRD). The gel decomposes in two stages. The first step involves evolution of water and ammonia at around 200^â—¦C and in the second step at around 600^â—¦C the decomposition of trapped formaldehyde (HCHO)occurs simultaneously with crystallization of zirconia. The crystallite size of the decomposed powder was found to be smaller than 10 nm.The decomposed powder could be sintered to around 90% of theoretical density using microwave energy source in 60 min.