A process for oligomerizing propylene is described, consisting of reacting said olefin in the presence of an X-ray amorphous alumina-silica gel catalyst with a silica/alumina molar ratio of between 30/1 and 500/1, a surface area of between 500 and 1000 m.sup.2 /g, a total pore volume of between 0.3 and 0.6 ml/g, an average pore diameter of the order of 10 .ANG. or less, and free or substantially free of pores with a diameter greater than 30 .ANG..