A study of the cationic reactivity of olefins present in the C-5 fraction has been performed under conditions similar to those used for synthesis of hydrocarbon resins. The most important olefins present in the C-5 fraction, viz. 2-methyl-l-butene, 2-methyl-2-butene, 1-pentene, cyclopentene, cis-2-pentene, trans-2-pentene, and 3-methyl-l-butene, were considered. Their relative reactivities were evaluated by using cyclopentene as a standard reference monomer in a series of copolymerizations at 40 degreesC in cyclohexane solution in the presence of AICl(3)-HCl-xylene complex as catalyst. Owing to the very low molecular weight values of the resulting oligomers, the systems were considered as copolymerizations in which transfer to monomer is operative. A copolymerization equation including monomer transfer was proposed from which the R parameters, related to the propagation and chain-transfer reaction, were obtained. The equation, by using the KelenTudos treatment, fits very well the experimental results, and the R values were obtained for the various pair of monomers. The system show almost ideal behavior, and a reactivity order for the olefins was obtained.