The direct synthesis of methanesulfonic acid (MSA) from CH4 and SO3 has been successfully achieved in an H2SO4 solvent using K2S2O8 as an initiator. The maximum SO3 conversion to MSA was achieved at 1000 psi of CH4, 30 wt % of SO3, and 350 mu mol of K2S2O8 at 333 K for a 2-h reaction time. SO3 conversion to MSA is approximately independent of reaction time above 2 h, at which time the SO3 conversion is observed to plateau at conversions much less than 100%. This plateau has been attributed to a loss of solution SO3 as a result of the formation of CH3(SO3)(n)H and (CH3SO2)(2)O. The rate of MSA formation exhibits a positive-order dependence on the CH4 pressure, and this pressure dependence is nonlinear, rising more rapidly for pressures equal to or greater than 800 psi. Addition of SO3 to the solvent medium at concentrations greater than 10% results in a rapid decrease in SO3 conversion as a result of the formation of significant byproducts and CH3(SO3)(n)H. At both 333 and 363 K, the SO3 conversion to MSA passes through a maximum with increasing K2S2O8 concentration, and then declines to a plateau for higher K2S2O8 concentrations. The position of the maximum is temperature-dependent and occurs at 100 mu mol of K2S2O8 at 363 K and 350 mu mol at 333 K. A maximum in SO3 conversion to MSA is also observed at 333 K when the temperature is increased from 313 to 363 K. A free radical mechanism has been proposed to account for the observed phenomena.