The oligomerisation of ethylene in excess hydrogen and trace CO over a Co/SiO2 catalyst has been investigated. Higher hydrocarbons were produced with yields up to 50%. The catalyst deactivated continuously but the selectivity to C-3+ hydrocarbons was largely independent of conversion. cis-2-Butene dominated amongst the oligomers formed initially. It reacted further to other butenes, n-butane, C-3, C-5, C-6, etc. when the ethylene conversion was near complete. Trace CO greatly inhibited the reaction at temperatures below 120 degrees C but increased the selectivity to higher hydrocarbons. The latter behaviour can be interpreted on the basis that CO competes with hydrogen thus disfavouring hydrogenation to ethane, which requires surface hydrogen, over dimerisation, which does not. The catalyst could be regenerated repeatedly through burn-off of carbonaceous material, which occurred near 180 degrees C with simultaneous oxidation of cobalt, necessitating a subsequent re-reduction. (C) 2013 Elsevier B.V. All rights reserved.