The geometries, harmonic vibrational frequencies, relative energetics, and enthalpies of formation of (CH3IO3) isomers and the reaction CH3O2 + IO have been investigated using quantum mechanical methods. Optimization has been performed at the MP2 level of theory, using all electron and effective core potential, ECP, computational techniques. The relative energetics has been studied by single-point calculations at the CCSD(T) level. Methyl iodate, CH3OIO2, is found to be the lowest-energy isomer showing particular stabilization. The two nascent association minima, CH3OOOI and CH3OOIO, show similar stabilities, and they are considerably higher located than CH3OIO2. Interisomerization barriers have been determined, along with the transition states involved in various pathways of the reaction CH3O2 + IO.