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The reactions of thioformaldehyde (H 2 CS) with OH radicals and assisted by a single water molecule have been investigated using high level ab initio quantum chemistry calculations. The H 2 CS + ˙OH reaction can in principle proceed through: (1) abstraction, and (2) addition pathways. The barrier height for the addition reaction in the absence of a catalyst was found to be −0.8 kcal mol −1 , relative to the separated reactants, which has a ∼1.0 kcal mol −1 lower barrier than the abstraction channel. The H 2 CS + ˙OH reaction assisted by a single water molecule reduces the barrier heights significantly for both the addition and abstraction channels, to −5.5 and −6.7 kcal mol −1 respectively, compared to the un-catalyzed H 2 CS + ˙OH reaction. These values suggest that water lowers the barriers by ∼6.0 kcal mol −1 for both reaction paths. The rate constants for the H 2 CS⋯H 2 O + ˙OH and OH⋯H 2 O + H 2 CS bimolecular reaction channels were calculated using Canonical Variational Transition state theory (CVT) in conjunction with the Small Curvature Tunneling (SCT) method over the atmospherically relevant temperatures between 200 and 400 K. Rate constants for the H 2 CS + ˙OH reaction paths for comparison with the H 2 CS + ˙OH + H 2 O reaction in the same temperature range were also computed. The results suggest that the rate of the H 2 CS + ˙OH + H 2 O reaction is slower than that of the H 2 CS + ˙OH reaction by ∼1–4 orders of magnitude in the temperatures between 200 and 400 K. For example, at 300 K, the rates of the H 2 CS + ˙OH + H 2 O and H 2 CS + ˙OH reactions were found to be 2.2 × 10 −8 s −1 and 6.4 × 10 −6 s −1 , respectively, calculated using [OH] = 1.0 × 10 6 molecules cm −3 , and [H 2 O] = 8.2 × 10 17 molecules cm −3 (300 K, RH 100%) atmospheric conditions. Electronic structure calculations on the H 2 C(OH)S˙ product in the presence of 3 O 2 were also performed. The results show that H 2 CS is removed from the atmosphere primarily by reacting with ˙OH and O 2 to form thioformic acid, HO 2 , formaldehyde, and SO 2 as the main end products.