The structure and chemical identity of ethylene van der Waals cluster ions, (C2H4)(n=2-9)(+), have been studied using triple quadrupole mass spectrometry. The collision-induced dissociation mass spectra reveal that cluster ions with n = 2-5 may undergo uncatalyzed cationic polymerization to form covalently bonded cationic oligomers (i.e., CnH2n+, n = 2-5). This has been confirmed by comparison of ethylene cluster ion mass spectra to those of known covalently bonded ions. Cluster ions with n > 5 consist of C10H20+ oligomer cores solvated by an appropriate number of ethylene monomers. Uncatalyzed cationic polymerization of ethylene appears to cease beyond n = 5 because of a "kinetic bottleneck". As the proposed structures of the n = 2-4 cluster ions are either linear or simple cyclic while the n = 5 ion is proposed to be branched, this kinetic effect is likely due to inhibition of further intracluster polymerization by steric hindrance.