A catalyst for the oligomerization of olefins, especially ethylene contains: at least one nickel complex e.g. Li.sub.2 Ni X.sub.2 that results from bringing into contact a nickel salt e.g. a nickel carboxylate with a bidentate chelating ligand containing a nitrogen-containing heterocyclic compound with an alcohol e.g. Ha-C(OH)RR'; and at least one hydrocarbylaluminum compound from tris(hydrocarbyl)aluminum compounds, chlorinated or brominated hydrocarbylaluminum compounds or at least one aluminoxane.