The present invention provides for a catalyst composition which is effective for use in the production of dimer products and higher olefin products from lower olefins such as propylene and butene in high yields and with an average degree of branching in the dimer products of less than about 1.6 methyl groups per molecule, generally in the range of from about 1.0 to 1.4 methyl groups per molecule. The present invention also provides a process for producing such dimer and higher olefin products using the catalyst composition of this invention. The catalyst of the invention comprises an amorphous nickel oxide (NiO) present as a disperse substantial monolayer on the surfaces of a silica (SiO.sub.2) support, which support also contains minor amounts of an oxide of aluminum, gallium or indium such that the ratio of NiO to metal oxide present in the catalyst is within the range of from about 4:1 to about 100:1. The catalyst may be prepared by precipitating a water insoluble nickel salt onto the surfaces of a silica support which has been impregnated with the metal oxide or onto a silica-alumina support which has been dealuminized such that the resulting nickel oxide/alumina ratio will fall within the ranges set forth above. The catalyst may then be activated by calcination in the presence of oxygen at a temperature within the range of from about 500.degree. to 700.degree. C.