Abstract: Methylation reactions of gaseous elementary mercury by halogen containing molecules such as halogenomethane species CH3X (with X = Cl, Br, and I) and the dimethylchlorinium ion CH3ClCH3+ were investigated at the density functional level. With CH3X, the reaction is predicted to be almost athermic and kinetically demanding for a thermal reaction. The reaction can proceed photochemically in the visible range; therefore sunlight may increase the reaction rate. These results compare well with the experimental data. Consecutive methylation of the CH3HgX products (with X = Cl, Br, and I) and subsequent formation of CH3HgCH3 were also studied. These reactions are predicted to be kinetically inaccessible and thermodynamically unfavorable. With CH3ClCH3+, the reaction is predicted to be athermic but kinetically easy. This is due to the suitability of the methyl transfer reagent. Geometrical and electronic data were systematically analyzed in order to rationalize the results.