The dynamics of initial decomposition of gas phase 1,3,3-trinitroazetidine (TNAZ) is investigated by using B3LYP/6-31+G(d,p) method and microcationical variational transition state theory. Three initial decomposition pathways, NO2 fission, HONO elimination, and direct ring-opening reaction are considered. The calculated rate constants indicate that the rate constants of NO2 fission reaction are larger than those of both HONO elimination and ring-opening reaction in gas phase. However, HONO elimination and ring-opening channels may be important for the decomposition reaction of TNAZ in solid state. These results provide new understanding of the dynamics of the initial decomposition steps of TNAZ. (c) 2005 Elsevier B.V. All rights reserved.