Complexes of general formula (eta(5)-L)RuCl(PR3)(2) are shown to be active and highly selective catalysts for the olefination of aldehydes in the presence of phosphines and diazoacetate at moderate temperatures. With equal catalyst loadings of Cp*RuCl(PR3)(2) shows comparable activity and higher selectivity with regard to the most active catalysts known to date for this reaction. Spectroscopic investigations demonstrate that the reaction mechanism includes the quantitative formation of the corresponding phosphorus ylide from the preformed phosphazine. (C) 2004 Elsevier Inc. All rights reserved.