AbstractSulfonation of anthracene‐9‐d with sulfur trioxide‐dioxane in dioxane as solvent at 40° leads to 31 ± 2% α‐, 8 ± 2% β‐ and 61 ± 2%meso‐substitution. The substitution at themeso‐position proceeds with a maximal substrate kinetic isotope effect of hydrogen ‐kH/kD= 6.8 with 5.6 and 8.6 as limiting values ‐ indicating that the proton removal from the σ‐complex is rate‐limiting.From the data for anthracene‐9‐d it was calculated that the sulfonation of C14H10leads to 21 ± 3% 1‐, 6