The invention relates to an olefin oligomerization process comprising the steps of: i) contacting a feed comprising olefins and nitriles with a guard bed comprising eta alumina; and ii) contacting the feed obtained in step i) with an oligomerization catalyst under conditions suitable to oligomerize the olefins in the feed. It also relates to the use of a guard bed comprising eta-alumina for reducing the content of basic organic compounds in a hydrocarbon feed.