The reaction rate and the ethylene/ethane product ratio in CO hydrogenation increase when cobalt wire, used as a catalyst for the reaction, is reduced to lower extents. The kinetic results obtained with the poorly reduced wire correlate well with temperature-programmed surface reaction(TPSR) and H₂ chemisorption results, and are interpreted to be due to electronic modification of the cobalt surface by the unreduced sub-surface cobalt oxide. It has been concluded that the extent of metal reduction is an important variable in determining the catalytic property of cobalt.