The gas-phase ion-molecule reactions of C-n(+) and C-n(2+) (n = 54, 56, 58, 60, and 70) with isobutene were investigated by utilizing a triple quadrupole mass spectrometer. In addition, the ion chemistry of C-60(+) and C-60(2+) with propene and 1-butene was investigated. The various ion-molecule reactions observed were highly dependent on the gas cell pressure and the collision energy of the primary ions. The product ions observed were governed primarily by associative and partial charge transfer reactions. The isomeric form of C4H8 as well as the size of the fullerene ion affected the reactions observed. For example, 1-butene readily adds to C-60(2+), while isobutene adducts were not detected. Also, fullerene cations with adjacent five-membered rings (n = 54, 56, and 58) react with isobutene to produce C-n(C4H8)(+) while C-60(2+) and C-70(2+) do not.