The unimolecular dissociation dynamics of selected alkene cluster ions (ethene, propene, 1-butene, trans-2 butene, and cis-2-butene) and their fast fragment ions has been investigated using a reflectron time-of-flight mass spectrometer. These ions were prepared by the supersonic expansion of a premixed sample gas containing 20% alkene in Ar, followed by multiphotoionization (MPI) using a Ti:sapphire femtosecond laser. From the unimolecular decomposition patterns of all the ions investigated, we conclude that after photoionization a radical cation initiated intracluster polymerization takes place in these cluster ions. But, due to steric hindrance, it does not continue indefinitely as the cluster size increases. For ethene, pentamerization can be observed, and for propene, trimerization; while in the case of the three isomeric butenes, 1-butene is partially trimerized and only dimerization is observed in 2-butene.