Upon thermal treatment, NaN3-doped zeolite transforms stepwise. After endothermic dehydration, the NaN3 decomposes exothermically in air and under N-2 flow. The temperature of decomposition is higher than that of the pure azide. Part of the sodium azide remains intact up to 1000 K. The process was also monitored by IR spectroscopy. Na-23 NMR measurements revealed that metallic sodium clusters were formed.