The geometries, harmonic vibrational frequencies, and relative energetics of CH3ClO3 isomers have been examined using quantum mechanical methods. At the G2MP2 level of theory, the lowest-energy structure is found to be the CH3OClO2 form followed by the chain-type CH3OOOCl isomer located 7.9 kcal mol(-1) higher. The QCISD(T) method predicts that the CH3OOOCl structure is located lower than CH3OClO2. The two higher-energy structures are CH3OOClO and CH3ClO3 across all energy levels. Interisomerization transition states have been determined, and the role of the formation of CH3ClO3 structures in the various pathways of the reaction CH3O2 + ClO is also examined.