Direct sulfonation of methane with SO3 to methanesulfonic acid (MSA) and methylbisulfate was investigated in sulfuric acid using Hg(I), Hg(II), and Rh(III)-salts as catalysts. O-2 was found to be an important component for the regeneration of the catalyst. The effects of O-2 pressure, temperature, SO3 concentration, methane pressure, and catalyst concentration were examined on the rates of SO3 conversion to MSA and methylbisulfate. The results of this investigation show that after 5 h at 148 degreesC using fuming sulfuric acid as the solvent, 360 psig CH4 reacts with 21 mmol of SO3 to give a 44% conversion of SO3 to MSA in the presence of 40 psig O-2 and 0.075 mmol of Hg(CF3SO3)(2) as the catalyst. The corresponding MSA selectivity is 87%. Conducting the reaction at higher temperature, higher O-2 pressure and higher initial concentration of SO3, leads to a reduction of the selectivity to MSA and an increase in the selectivity to CH3OSO3H. A mechanism for the formation of MSA and CH3OSO3H is proposed in the light of the experimental results. (C) 2003 Elsevier B.V. All rights reserved.