Olefinic feedstocks which contain catalyst contaminants or poisons such as sulfur-containing compounds may be oligomerized to a desired oligomer and configuration of the oligomer by utilizing a catalyst which is stable and will not deactivate due to the presence of the aforementioned poisons. The catalyst comprises a porous support which has been impregnated with a catalytically effective amount of an iron group metal compound and a Group IVA metal compound. In addition, the catalyst composite may also contain in combination therewith, a catalytically effective amount of an alkyl aluminum compound and an aluminum alkoxy/aluminum halide compound. The catalyst of this invention is prepared by impregnating a porous support with an aqueous solution containing an iron group metal salt and a Group IVA metal salt, calcining and containing the calcined support with a solution containing an alkyl aluminum compound.