The cycloadditions of cyclopentadiene with diphenylketene and dichloroketene are studied by a combination of kinetic and product studies, kinetic isotope effects, standard theoretical calculations, and trajectory calculations. In contrast to recent reports, the reaction of cyclopentadiene with diphenylketene affords both [4+2] and [2+2] cycloadducts directly. This is surprising, since there is only one low-energy transition structure for adduct formation in mPW1K calculations, but quasiclassical trajectories started from this single transition structure afford both [4+2] and [2+2] products. The dichloroketene reaction is finely balanced between [4+2] and [2+2] cycloaddition modes in mPW1K calculations, as the minimum-energy path ( MEP) leads to different products depending on the basis set. The MEP is misleading in predicting a single product, as trajectory studies for the dichloroketene reaction predict that both [4+2] and [2+2] products should be formed. The periselectivity does not reflect transition state orbital interactions. The C-13 isotope effects for the dichloroketene reaction are well-predicted from the mPW1K/6-31+G** transition structure. However, the isotope effects for the diphenylketene reaction are not predictable from the cycloaddition transition structure and transition state theory. The isotope effects also appear inconsistent with kinetic observations, but the trajectory studies evince that nonstatistical recrossing can reconcile the apparently contradictory observations. B3LYP calculations predict a shallow intermediate on the energy surface, but trajectory studies suggest that the differing B3LYP and mPW1K surfaces do not result in qualitatively differing mechanisms. Overall, an understanding of the products, rates, selectivities, isotope effects, and mechanism in these reactions requires the explicit consideration of dynamic trajectories.