A process is described in which an unsaturated fat is reacted with ethylene in a metathesis reaction in the presence of at least one non-aqueous ionic liquid to produce both an olefinic fraction and a composition of monoalcohol or polyol esters.Particular application to an oleic sunflower seed oil, an oleic rapeseed oil or to a mixture of monoalcohol esters of said oils, the process producing both an olefinic fraction and a monoalcohol or glycerol esters composition generally having more than half of its chains constituted by unsaturated C.sub.10 chains.