Hydrooligomerization or polymerization of cyclopentene using racemic 1,2-ethylenebis(eta(5)-indenyl)zirconium dichloride (1) and methylaluminoxane leads to the production of poly(cis-1,3-cyclopentane). Analogous reactions using racemic 1,2-ethylenebis(eta(5)-tetrahydroindenyl)zirconium dichloride (2) leads to the production of oligomers in which cyclopentene is incorporated in a cis- or trans-1,3 manner. Two plausible mechanisms for trans-1,3 insertion are presented which involve reversible beta-hydrogen elimination reactions of unsaturated oligomers or direct interconversion of olefin hydride complexes via the intermediacy of a sigma-CH complex. Copolymerization of cyclopentene and 3-cyclopentylcyclopentene-2-d (9) reveals that the former process can occur but is not competitive with simple copolymerization; this, in combination with other observations, suggests that reversible chain transfer is unlikely to account for trans-1,3 insertion. Polymerization of cyclopentene-d(8) led to significantly different, instantaneous ratios of cis to trans trimers compared with that observed during polymerization of cyclopentene. The observed deuterium isotope effect on stereochemistry, in combination with other work, can be interpreted in terms of trans-1,3 insertion occurring predominantly via the second mechanism.