An olefin metathesis process involving contacting an unsaturated fatty acid ester or unsaturated fatty acid, such as methyloleate or oleic acid, with a lower olefin, preferably ethylene, in the presence of a metathesis catalyst so as to prepare a first product olefin, preferably, a reduced chain .alpha.-olefin, such as methyl-9-decenoate or 9-decenoic acid, respectively. The metathesis catalyst contains ruthenium or osmium and a chelating ligand, preferably, a chelating ligand containing a carbene moiety and a second donor moiety of a Group 15 or 16 element Optionally, the catalyst can be supported on a catalyst support, such as, a cross-linked polymeric resin.