This paper reports on computational studies of gas-phase reactions of SiO and Si 2O 2. The oxidation of SiO can initiate efficient formation of silica or silicate dust particles in a wide range of environments. Both OH radicals and H 2O molecules are often present in these environments, and their reactions with SiO and the smallest SiO cluster, Si 2O 2, affect the efficiency of eventual dust formation. Density functional theory calculations on these reactions, benchmarked against accurate coupled cluster calculations, indicate that the Si 2O 2 + OH reaction should be faster than SiO + OH. The reaction SiO + H 2O → SiO 2 + H 2 is both endothermic and has high activation energies to reaction. Instead, the formation of molecular complexes is efficient. The reaction of Si 2O 2 with H 2O, which has been suggested as efficient for producing Si 2O 3, might not be as efficient as previously thought. If the H 2O molecules dissociate to form OH radicals, oxidation of SiO and Si 2O 2 could be accelerated instead.