In a feed clean-up process at least two adsorbents (2, 4) are installed in front of an oligomerization reactor (3). Olefin feed is sent over one adsorbent (2) and the nitrile poisons are adsorbed so that clean feed will enter the reactor (3). Before the adsorbent (2) will be saturated, the feed (1) is sent to the other, fresh adsorbent (4). At the same time oligomerization product from the reactor (3) is used to desorb nitriles from the spent adsorbent (2).