Zinc metal was shown to considerably increase the activity of the metal(III) chloride salts of ruthenium, iridium and osmium as metathesis initiators for the ROMP of norbornene. The stereochemistry of the polymers formed was determined by H-1 and C-13 NMR. Formation of the initial carbene species is not via the normally accepted vinyl hydride mechanism. An alternative mechanism involving a metallacyclopentane intermediate is postulated. (c) 2006 Elsevier B.V. All rights reserved.