The highly efficient activation of methane by a homogeneous mercury(II) catalyst has been investigated by an ab initio computational study of the interaction of methane with the HgF+ ion. The key steps in the activation were found to be the formation of a loose adduct between methane and the HgF+ ion followed by abstraction of a proton by a solvent molecule. The efficiency of the methane activation is enhanced by the high solvation energy of the proton and the formation of the strong HgCH3 bond.