The effect of carbon monoxide on the selective hydrogenation of acetylene over a 2% Ni/SiO2 catalyst at temperatures below 100 degrees C has been investigated. The product distribution is constant over a wide range of conversions. In the absence of carbon monoxide approximately two-third of the acetylene which reacts is converted to C-2 products with an ethane to ethylene ratio of 1:2. The addition of carbon monoxide lowers activity, with a kinetic order of -0.2, decreases the selectivity to C-2 products and decreases the extent of ethane formation. These trends are consistent with a parallel reaction scheme in which CO lowers the availability of hydrogen on the nickel surface. Further hydrogenation of product ethylene to ethane, which is intrinsically much faster than that of acetylene, is minimal for acetylene conversions below 99%. However, it does occur with feeds containing excess ethylene. The catalyst shows slow, continuous deactivation during repeated reactions. The deactivation is not due to migration of nickel, as would be expected if Ni(CO)(4) was formed. It is associated with the build-up of carbonaceous matter in amounts greatly exceeding the number of surface nickel atoms but less than that known to cause loss of activity through blockage of pores. (c) 2009 Elsevier B.V. All rights reserved.