The cationic polymerization of 1,3-pentadiene isomers initiated by AlCl3 was investigated in nonpolar solvent. It was shown that the polymerization was very sensitive to the initial monomer composition (proportion between the cis and trans isomers). The cis isomer favors the cross-linking reactions and double bond isomerization, whereas the trans isomer induces more cyclization. However, the behavior of the mixture of the two isomers cannot be simply deduced from a linear combination of the behavior of each isomer independently considered. Some polymerizations were realized in different alkanes (pentane, hexane, heptane, and decane). Despite similar polarity, it was observed that the higher the solvent molecular weight, the lower the total polymerization yield. These results were assigned to a different solubility of the Lewis acid and of polymer chains in the different solvents investigated. Finally, two origins of the cross-linked polymer were identified: the heterogeneity of the polymerization medium and intermolecular reactions of the active species with the polymer.