The direct and selective oxidation of methane to methyl bisulfate in oleum is reported. Iodine is used as a catalyst. In order to optimize the process the initial methane pressure, oleum concentration and temperature are varied. The reaction is first order with methane pressure and sulfur trioxide concentration. The rate of the ester production is defined by the empirical equation in the temperature range 80-180 degrees C. methane pressure 1-5 MPa, sulfur trioxide concentration 0-5.95 mol dm(-3), time 0-20 h. The equation fits the experimental data well. The apparent activation energy of methane oxidation in oleum with iodine as a catalyst is found to be equal to 44.8 +/- 1.2 kJ mol(-1). (C) 2009 Elsevier B.V. All rights reserved.