When several p-benzoquinones in methylene chloride solution are mixed at room temperature with RuCl2(PCy3)(2)(CHPh), RuCl2(PPh3)(3), RhCl(PPh3)(3) and CoBr2(PPh3)(2), strong, well-resolved, persistent epr signals are observed, consistent with the formation of radical anions. In the case of the Grubbs' catalyst, RuCl2(PCy3)(2)(CHPh), evidence is presented that these signals arise from charge transfer. The Grubbs' catalyst, is also remarkable in that similar epr activity is observed using norbsmadienes, and to a lesser extent during and after metathesis polymerisation of norbornene and cyclopentene. The dienes are also much more efficient promoters of initiation of these polymerisations than the monoenes, and evidence is presented that the presence of p-benzoquinone enhances the reactivity of the propagating metallacarbenes. (C) 2000 Elsevier Science S.A. All rights reserved.