A variety of ruthenium-allenylidene complexes were produced in two steps from the simple RuCl2(dmso)(4) precursor: the neutral RuCl2(=C=C=CR2)(PCy3)(n)(dmso)(m) and the cationic [RuCl(=C=C=CR2)(PCy3)(n)(dmso)(m)]TfO complexes. Some of them have been evaluated for polymerisation of norbornene and cyclooctene, and constitute the first example of active Ru=C=C=CR2 catalysts for ring opening metathesis polymerisation (ROMP). The most active system is the cationic [RuCl(=C=C=CPh2)(PCy3)(dmso)(2)]TfO complex 5 which leads in PhCl at 80degreesC to 90% of polycyclooctene with a polydispersity of 1.89 and 69% of cis-configuration of CH=CH bonds. The presence of an electron-donating group on the aryl substituents (p-OMe) drastically favours both yield and low polydispersity of the polymer. (C) 2002 Elsevier Science B.V. All rights reserved.