The invention provides an olefin oligomerization process comprising the steps of: i) reducing the level of nitriles in an olefin feed by contacting the feed with a guard bed comprising gamma alumina having a surface area greater than 250 m2/g; and ii) contacting the olefin feed obtained in step i) with an oligomerization catalyst under conditions suitable to oligomerize the olefins in the feed; wherein the gamma alumina used in step i) has been obtained from needle shaped boehmite.