The cationic copolymerizations of α-methylstyrene and cyclopentadiene were carried out and the monomer reactivity ratios were determined. It was found that cyclopentadiene was more reactive than α-methylstyrene, and that the effect of solvent polarity on the monomer reactivity ratio was small. It was deduced from the pKBH+values of cations pertinent to the present investigation that there is little difference between the stabilities of cyclopentenyl cation and cumyl cation. The present experiment seems to be the first example showing that there is little solvent effect even in the copolymerization between monomers of dissimilar structures, if the stabilities of cations are similar.