Novel water-soluble ruthenium carbene complexes with finely tuned structure and properties in solution are reported. These ruthenium-based initiators were found to exhibit great catalytic activity in aqueous miniemulsion ring-opening metathesis polymerization of norbornene. Stable particles of polynorbornene could be generated in the 200-250 nm size range stabilized with a nonionic surfactant (polystyrene-b-poly(ethylene oxide)). (c) 2006 Wiley Periodicals, Inc.