The thermal decomposition of CH2N2, highly diluted in Kr, has been studied in shock waves by using time-of-flight mass spectrometry to follow concentration changes in the reacting gas. Observations were made at pressures 45 to 95 Torr and temperatures 820 to 1200°K. The principal products of reaction are C2H4, C2H2, and N2. The primary step appears to be a second order unimolecular decomposition of CH2N2into CH2and N2; for this step, logk = (9.61 ± 0.21) − (15 800 ± 1 000)/2.303RT(cal mole−1and cm3mole s units.) Some decomposition of CH2N2into HCN and NH is also indicated. The formation of C2H2is believed to occur through vibrationally excited C2H4, formed by reaction between CH2and CH2N2. Calculations using the R.R.K.M. theory indicate that 50 + 10% of the energy of this reaction appears as vibrational energy of the product C2H4.