The effect of small amounts of carbon monoxide on the oligomerization of acetylene over a low loaded Ni/SiO2 catalyst has been investigated. Although CO does not participate in the reaction it has two beneficial effects if oligomer products are desired. Conversion of acetylene is higher. both initially and over time, and the selectivity to C-4-C-10 product is improved relative to C-2 hydrogenation. Higher products are predominantly aliphatic alkenes. CO increases formation of branched products relative to linear ones and dienes relative to alkenes while Suppressing benzene and higher aromatics. In the absence of CO, unsaturated products can be largely hydrogenated to alkanes if a Pt/SiO2 catalyst is located immediately downstream the Ni/SiO2 but the hydrogenation of dienes and trienes proceeds only as far as alkenes when the feed contains CO. The twin effects of CO can be interpreted on the basis that CO competes for nickel sites that would otherwise be able to accept hydrogen atoms from H-2 or C2H2. The Ni/SiO2 catalyst was sensitive to feed impurities such as phosphine present in some grades of acetylene but deactivation was not complete. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.