The electrophilic sulfonation of several arenes with SO3 was examined by electronic structure computations at the M06-2X/6-311 +G(2d,2p) and SCS-MP2/6-311+G(2d,2p) levels of theory. In contrast to the usual interpretations, the results provide clear evidence that in nonpolar media and in the absence of catalysts the mechanism of aromatic sulfonation with a single SO3 is concerted and does not involve the conventionally depicted 1:1 c complex (Wheland) intermediate. Moreover, the computed activation energy for the 1:1 process is unrealistically high; barriers for alternative 2:1 mechanisms involving attack by two SO3 molecules are 12-20 kcal/mol lower! A direct 2:1 sulfonation mechanism, involving a single essential transition state, but no Wheland type intermediate, is preferred generally at MP2 as well as at M06-2X in isolation (gas phase) or in noncomplexing solvents (such as CCl4 or CFCl3). However, in polar, higher dielectric SO3-complexing media, M06-2X favors an SEAr mechanism for the 2:1 reaction involving a Wheland-type arene-(SO3)(2) dimer intermediate. The reaction is slower in complexing solvents, since the association energy, e.g., with nitromethane, must be overcome. But, in accord with the experimental kinetics (second-order in SO3), attack by two sulfur trioxide molecules is still favored energetically over reaction with a single SO3 in CH3NO2. The theoretical results also reproduce the experimental reactivity and regioselectivity trends for benzene, toluene, and naphthalene sulfonation accurately.