¹ H and ¹³C NMR studies have shown that the long-lived 9,9-dimethyl-10-R-phenanthrenium cations (R = PhCC, Me, OH) generated in FSO₃H–SbF₅/SO₂ClF/SO₂/CD₂Cl₂ transform into long-lived 7-dihydroxysulfonio-9,9-dimethyl-10-R-phenanthrenium dications. The effect of the 10-R substituents on the reaction rate suggests that a key step in the reaction mechanism is addition of SO₂ to protonated phenanthrenium cations.