Complex OsHCl(CO)(P i Pr 3 ) 2 catalyzes the ring-opening metathesis polymerization (ROMP) of norbornene and 2,5-norbornadiene to give poly(norbornene) and poly(norbornadiene), respectively. In both cases the resulting polymers have a high cis (74-95%) content. The stereoregularity or tacticity of the cyclopentane and cyclopentene ring sequences in poly(norbornene) and poly(norbornadiene) estimated from the 1 3 C{ 1 H} NMR spectra of the hydrogenated derivatives was found to be syndiotactic. Complex OsHCl(CO)(P i Pr 3 ) 2 is also active in tandem ROMP-hydrogenation of norbornene and 2,5-norbornadiene. At 40 o C and 3 atm of H 2 , poly(norbornene) is fully hydrogenated in 48 h, while poly(norbornadiene) is fully hydrogenated in 48 h at 75 o C and 3 atm of H 2 . The complex RuHCl(CO)(P i Pr 3 ) 2 is also active in ROMP and tandem ROMP-hydrogenation of norbornene, obtaining trans-poly(norbornene) and hydrogenated poly(norbornene), respectively.