A detailed study of the microstructures of ring-opened polymers of norbornene and several methyl derivatives. Using salts of Ru and Os as catalysts, show that the widely noted stereoselective feature of cis and trans giving trans in olefin metathesis is attributable to different forms of metallacarnene propagating species rather than to different configurations of putative metallacyclobutanes derivable from the same metallacarbene.