Diaz-Urrutia and Ott (Reports, 22 March 2019, p. 1326) report a selective conversion of methane to methanesulfonic acid that is proposed to occur by a cationic chain reaction in which CH3+ adds to sulfur trioxide (SO3) to form CH3-S(O)(2)O+. This mechanism is not plausible because of the solvent reactivity of CH3+, the non-nucleophilicity of the sulfur atom of SO3, and the high energy of CH3-S(O)(2)O+.