Results of an experimental study using the selected-ion flow tube (SIFT) technique are reported for ion-molecule reactions of C-60(.+) With a variety of cycloalkenes and acyclic and cyclic dienes at 294 +/- 2 K in helium gas at a pressure of 0.35 +/- 0.01 Torr. Addition is observed only with 1,3-cyclopentadiene and 1,3-cyclohexadiene. Rate coefficients were measured to be 1.0 and 1.5 x 10(-11) cm(3) molecule(-1) s(-1), respectively. No reactions, k < 3 x 10(-12) cm(3) molecule(-1) s(-1), were observed with 1,3-butadiene, isoprene, 1,3-pentadiene, cyclopentene, cyclohexene, furan, and 1,4-cyclohexadiene. These results provide indirect evidence for the occurrence of a Diels-Alder cycloaddition with 1,3-cyclopentadiene and 1,3-cyclohexadiene. The addition reactions of these two molecules with C-60(.+) were found to be 5 +/- 1 and 6 +/- 2, respectively, faster than the addition reactions with C-70(.+) for which rate coefficients of 2.0 and 2.5 X 10(-12) cm(3) molecule(-1) s(-1), respectively, were measured. This relative reactivity, which in the case of 1,3-cyclopentadiene is remarkably similar to a reported relative reactivity of 7:1 measured in toluene solution at 293 K, has been interpreted in terms of the hybridization of the C atoms in C-60 and C-70.