Using the CCSD(T)/cc-pVDZ//B3LYP/6-311G(2d,2p) method, we investigated the detailed potential energy surfaces for the unimolecular dissociation reactions of chloromethyl peroxynitrate (CH2ClO2NO2). The results show that there are four most stable isomers of CH2ClO2NO2, named IS1, IS2, IS3 and IS4. From these isomers, twenty unimolecular decomposition reaction channels have been studied and discussed. Among them, the predominant thermal decomposition pathways are those direct O-N bond rupture to produce o-cis-CH2ClO2* + NO2 or i-cis-CH2ClO2* + NO2. The corresponding O-N bond dissociation energies (BDEs) are 22.0, 22.0, 21.7 and 21.7 kcal/mol, respectively, in good agreement with the experimental results. (C) 2014 Elsevier B.V. All rights reserved.