The cationic polymerization of 1,3-pentadiene (PD) initiated by AlCl3 in n-hexane was carried out in the presence of allylic halides, i.e., 3-chloro-1-butene, allyl chloride, and allyl bromide. All these allylic halides strongly hindered crosslinking which is a major side reaction during PD cationic polymerization. Cyclization, another side reaction, was also reduced by the addition of the allylic halides. The influences of these halides on polymer yields and molecular weights are also discussed.