The polymerizations of 2-methyl-2-butene (2M2B) were carried out by various cationic initiators in different media at 30 degrees C. All these polymerizations generate oily oligomers of rather low degrees of polymerization (3-5). Evidences from i.r. and H-1 n.m.r, show that the oligomer chain contains structural units derived from both 2M2B and 2-methyl-1-butene (2M1B), which indicates that 2M2B isomerizes partially to 2M1B during the polymerization. The unsaturated endgroups also arise from chain transfers of both 2M2B and 2M1B growing carbocations by proton-elimination. The reaction carried out in toluene fails to produce any polymer or oligomer but generates an adduct of 2M2B and toluene by Friedel-Crafts alkylation.