A series of ab initio calculations have been carried out to determine why the a,b- and cc-isomers are the most commonly observed mono-oxides Of C-70 in ozonolysis reactions, when existing calculations in the literature report that these structures are not the most stable conformations. We show that the a,b- and cc-isomers are the two most stable structures on the C70O3 potential energy surface, which suggests that the reaction pathway toward oxide formation must proceed via the corresponding ozonide structure. From our calculations, we offer a mechanism for the thermally induced dissociation Of C70O3 that share the first two steps with the general mechanism for ozonolysis of alkenes proposed by Criegee. We suggest further steps that involve C70O3 losing 0,) in its triplet or singlet state, thus leaving C70O in its triplet or singlet state, respectively. A pair of products in their singlet states seems to be more likely for the decomposition of a,b-C70O3, which ultimately leads to the closed a,b-C70O epoxide structure. For c,c-C70O3, the more thermodynamically favorable route is the triplet channel, resulting in the triplet open c,c-C70O oxidoannulene structure, which may subsequently decay to the singlet ground state c,c-C70O epoxide form. This finding offers an alternative interpretation of the experimental observations which reported an open d,d-C70O oxidoannulene structure as the metastable intermediate.