An exploratory study has been made on the SO, sulfonation of a number of alkylarenes in [D,]dichloromethane as an aprotic solvent, mostly in the presence of [D8]1,4-dioxane as a reactivity moderator, in order to obtain information on sulfonic acid and sulfonic anhydride formation. 1,2-Dimethylbenzene (1) with 1.0 mol-equiv. of SO3 yields predominantly the 4-sulfonic acid (1-4-S), whereas with 4.0 mol-equiv. of SO3, 18 mol-Yo of the corresponding (intermolecular) sulfonic anhydride ( 1-4-S02)20 is additionally formed. 1,4-Dimethylbenzene (2) with 0.8 mol-equiv. of SO3 yields predominantly 2-2-S, whereas with 4.0 mol-equiv. of SO, the sulfonic anhydride (2-2-S02)20 and its 6-S and 6,6'-S2 derivatives are formed, the eventual main product being (6-S-2-2-S02),0. Reaction of 1-ethyl-4-methylbenzene (3) with 2.0 mol-equiv. of SO3-[D,]dioxane at -78 + -40 "C leads to the predominant formation of the 2-and 3-S derivatives in a molar ratio of 26:74, with small amounts of the three corresponding intermolecular anhydrides. Upon performing the sulfonation of 3 at 0 + 22 "C, the relative amounts of the anhydrides are larger, and in addition the 3,5-s2 derivatives of the intermolecular anhydrides are formed. The absence of the isomeric 2,6-S2 derivative is ascribed to the buttressing effect of the 2-sulfo on the 1-ethyl group, hampering the sulfonation at C(6). Reaction of diphenylmethane (4) with 4.2 mol-equiv. of SO3 at -20 + 25 "C yields initially 4-4'-S and subsequently 4-4',4"-S2. Sulfonation of 1,2-diphenylethane (5) with 4.0 mol-equiv. of SO, at -78 + 22 "C for 4.5 h leads mainly to the cyclic dimeric disulfonic anhydride [5-4',4"(-S02)20]2, which upon hydrolysis affords 9% of 5-2',4"-and 91% of 5-4',4"-s2. However, sulfonation of 5 at 0 + 22 "C for ca. 0.8 h was found to lead to the formation of the intermolecular sulf onic anhydride (4 ' -S -5 -4 ' '-SOz)zO. On sulfonation of 1,2di-p-tolylethane ( 6 ) with 1.0 mol-equiv. of SO3, only ca. 20% of the substrate was sulfonated to give, after hydrolysis, 3% of the 2'-and 12% of the 3'-sulfonate, whereas with 4.0 molequiv. of SO3 the initial monosulfonic acids are almost completely converted to yield the 2',3",5"-s3, 3',3",5"-S3, and 3',3 ",5' ,5"-S,.