The reaction of sulfur trioxide with benzene, the four halogenobenzenes, and six halogenonaphthalenes in dichloromethane as solvent, and with four chlorinated anthracenes in nitromethane as solvent, has been studied by analysis of the resulting mixture of the sulfo derivatives with1H NMR. The sulfonation of benzene yields initially benzenesulfonic acid and subsequently the 1,3-disulfonic acid (1,3-S2). The initial sulfonation of the four halogenobenzenes yields ≥ 98% of the 4-S. Their subsequent sulfonation gives the 2,4-S2and in the case of fluoro- and iodobenzene also the 2,4,6-S3. Monosulfonation of 1-fluoronaphthalene yields only the 4-S, whereas the three other 1-halogenonaphthalenese yield in addition some 5-S. Further sulfonation on any of the four 1-halogenonaphthalene-4-sulfonic acids yields a mixture of the 2,4-S2and 4,7-S2, and eventually also some 2,4,7-S3, whereas the 1-halogeno-5-sulfonic acids give the corresponding 5,7-S2. Sulfonation of 2-chloro- and 2-bromonaphthalene yields initially 85% 8-S and 15% 4-S, which are subsequently converted into the 6,8-S2and 4,7-S2, respectively. On reaction with 1.0 mol-equiv. of SO3, 2-chloroanthracene gives the 9-S, and 9-chloroanthracene gives an 18:82 mixture of the 4-S and 10-S. Both 1,5- and 1,8-dichloroanthracene yield initially the 4-S and subsequently the 4,8-S2and 4,5-S2, respectively.