A new series of iodine-containing compounds is reported as catalysts for the direct and selective oxidation of methane to methyl sulfates (especially mono methyl sulfate) in 65 wt.% oleum. The solubility of elemental iodine in 65 wt.% oleum at 180 degreesC was found to be ca. 34 mmol/l with only the dissolved part of the iodine as the active catalyst. A mathematical model for the relationship between the initial reaction rate and the iodine concentration is given. It is found that the iodine-catalyzed reaction is first order for methane (up to above 100 bar), but 0.5 order for iodine. Suitable reaction temperatures are in the range 170-190 degreesC. Besides elemental iodine, many compounds containing iodine, such as KI, NaI, CH3I,I2O5, KIO3 and KIO4, also catalyze the methane oxidation to methyl sulfates in oleum in the same manner and at the same iodine concentration indicating that they are converted into the same catalytic intermediate. High performance liquid chromatography and gas chromatography and other methods was used to clarify the stoichiometry of the proposed reaction schemes. Titration results showed that, when methane was in excess compared to the amount of SO3 in the beginning of an experiment, almost pure sulfuric acid (99.06 +/- 0.15%) was the result in the end of the experiment. Gas chromatograph measurements showed that 4.6% of the consumed methane was converted to CO2 while the overall selectivity of methane to methyl sulfates was found to be ca. 92%. (C) 2004 Elsevier B.V. All rights reserved.