The initial decomposition products of 1,3,3-trinitroazetidine (TNAZ) are characterized using time-of-flight mass spectrometry. To induce thermal decomposition, solid samples of TNAZ are irradiated with 248 nm laser pulses. Products released to the gas phase are ionized by a pulse of 118 nm light, and the ions are detected using a time-of-flight mass spectrometer. Neutral time-of-arrival scans are used to determine the translational temperature and the relative yields of the products. The translational temperature is found to depend linearly on the decomposition laser fluence, consistent with a photothermal decomposition mechanism. Analysis of the temperature dependence of the relative product yields shows that two reactions are important in the initial decomposition of TNAZ : unimolecular nitro-nitrite rearrangement followed by loss of NO and a bimolecular reaction generating nitrosodinitroazetidine (NDNAZ). Loss of NO2 appears to be an early, but not initial, decomposition step. Experiments with isotopically labeled TNAZ show that the nitro-nitrite rearrangement occurs on the nitramine group and that NO is lost from both the nitroalkyl and nitramine groups. NDNAZ is generated by replacement of the leaving nitramine nitro group by NO.