The adsorption of acetylene on silica-supported nickel has teen investigated using a [14C]radiotracer technique. It has been found that the acetylene adsorption isotherms have two regions, a steep primary region and a linear secondary region. From the amount of ethane formed during the primary adsorption of acetylene, the average composition of the primary region has been calculated to be C2H1.9 indicating that the species adsorbed on the primary region are predominantly dissociatively adsorbed. Evidence has been obtained to show that the species involved in actual catalytic turnover during a hydrogenation reaction are located on the secondary region.