The rate constants and the yields of the products of the nucleophilic substitution and elimination reactions of 1-(4-methoxyphenyl)-3-methyl-3-butyl derivatives (1-X) have been determined in mostly aqueous solvents, and the absolute rate constant for reaction of the simple tertiary carbocation 1(+) in 50:50 (v/v) trifluoroethanol/water has been estimated as k(s) = 3.5 x 10(12) s(-1). Product studies show that the acid-catalyzed reactions of 1-OH and 4-(4-methoxyphenyl)-2-methyl-1-butene (2) do not proceed exclusively through a common carbocation intermediate 1(+). The observed formation of small amounts of the azide ion adduct 1-N-3 likely occurs by a preassociation reaction mechanism. The reactions of 1-Cl and 1-O2CC6F5 in 50:50 (v/v) trifluoroethanol/water give 39% and 56%, respectively, of the alkene products of elimination, and the yields of the alkenes are unaffected by the addition of 0.50 M of the good nucleophile N-3(-). This result is consistent with a concerted unimolecular mechanism for the elimination reactions of l-X.