The gas-phase cycloaddition reactions of ethylene and 1,3-butadiene with the cyclopropylcarbinyl cation to produce the cyclohexyl and cycloocten-5-yl cation, respectively, have been computationally characterized. At levels of theory up to MP2/cc-pVDZ the reactions are quite asynchronous but are concerted (i.e.; there is only one transition state prior to formation of the product carbocycles).