The formation of the chlorosiloxane ring (Cl2SiO)(2) from the reaction of SiCl4 with O-2 has been studied using density functional theory (DFT). Geometries of reactants, intermediates, transition states, and products were fully optimized, and the relative energies of the stationary points and all of the transition states were calculated on the B3LYP/6-311G* level. The initial reaction of O-2 with SiCl4 starts on the triplet surface with the insertion of O-2 in SiCl4. This will loosen one Cl atom, and a barrier of 56.6 kcal/mol must be overcome. In the next step Cl-2 is eliminated and the cyclic-Cl2SiO2 is formed. The latter reacts with SiCl4, which involves a barrier of 76.0 kcal/mol. This process leads to the formation of a low-lying intermediate Cl2SiO(OCl)SiCl3. The intermediate proceeds under elimination of Cl-2 to the product (Cl2SiO)(2) ring over a barrier of 71.3 kcal/mol. This study also shows that the formation of the Cl3SiO radical is found to be energetically more favorable than that of Cl2SiO. This can lead to the growth of larger chlorosiloxanes.