The molecular reaction pathways Of SO3 and SOCl2, reactions with one and two water molecules were studied using ab-initio and DFT quantum chemical methods (up to MP2/6-311++G(3df,3pd) and QCISD(T)/6-311++G(2df,2pd)) in order to describe the experimentally observed kinetic features Of SO3 hydration and SOCl2 hydrolysis in the gas phase and to elucidate the reaction mechanism providing the high reaction rate. The transition state structures were located, and absolute values of kinetic rate constants were estimated for a variety of possible reaction pathways including bi- and terinolecular S=O and S-Cl bond hydrolysis, chlorine shift to oxygen, dissociative addition with formation of hexavalent sulfur derivative, and adjacent reactions. It was found that the termolecular mechanism Of SO3 hydration proposed earlier by Morokuma and Muguruma is in good agreement with the experimental rate constants. However, none of the proposed molecular pathways of SOCl2, hydrolysis can provide the high reaction rate observed experimentally. Thus, the real hydrolysis mechanism of halogen derivatives of second-row elements in the gas phase can be sufficiently different from the regular molecular pathways, which are usually considered.