Abstract: The present invention concerns a process for oligomerizing an olefin feedstock to form an oligomerization product, and a method of controlling such an oligomerization process. The process comprises oligomerizing propylene to form a Cn olefin, including contacting a feed stream comprising propylene and a recycle fraction with a solid phosphoric acid oligomerization catalyst under effective oligomerization conditions in an oligomerization reactor to produce an oligomerization effluent; and fractionating the oligomerization effluent to obtain a product fraction and the recycle fraction, the product fraction comprising the Cn olefin and the recycle fraction comprising a Cn-3 olefin; wherein the recycle fraction comprises at least 80 wt % of the Cn-3 olefin, based on the weight of the recycle fraction; and wherein n is 9, or 12.