Cyclooctene is notable because it is the smallest cycloalkene that can exist stably as either the cis or trans stereoisomer, with cis-cyclooctene being the most common. Theoretical analysis implies a total of 16 conformational and configurational isomers, all chiral, forming 8 enantiomeric pairs. The cis isomer can adopt various conformations, the most stable one being shaped like a ribbon. The most stable conformation of trans-cyclooctene is shaped like the 8-carbon equivalent of the chair conformation of cyclohexane.
Longer cycloalkene rings such as the ten-carbon cyclodecene also occur as cis and trans isomers.