The kinetics of the reactions of the carbocations CH3[C(CH3)(2)CH2](n)C(+)Ph(2) (1a(+)-1d(+), n = 0, 1, 2, 34) with allyltrimethylsilane (4) and dimethylphenylsilane (5) have been investigated. It is found that the carbocation 1a(+) (n = 0) is 10(2) times more reactive than the more highly substituted analogues 1b(+)-1d(+), which hardly differ in their electrophilic reactivity. We, therefore, conclude that the (CH3)(3)CCH2 and (CH3)(3)CCH2C(CH3)(2)CH2 groups are suitable to mimic the polyisobutylene chain in a growing carbocation.