Infrared spectroscopy has been used to study the adsorption of carbon monoxide (358-493 K, 0.1-20 kPa) on four copper-on-silica (2-10 wt% Cu) catalysts prepared by the "ion-exchange" technique. The measurements are made for each sample in three different states: unreduced (predominantly Cu2+), reduced (Cu0), and partially reoxidised in nitrous oxide (Cu+). On unreduced samples, a major absorption band between 2127 and 2132 cm-1 due to CO adsorbed on small CuO particles and a weak band at 2199 cm-1 due to CO on isolated Cu2+ ions incorporated in the silica surface have been identified. The former adsorption obeys a Langmuir isotherm with a heat of adsorption of 29 kj/mol independent of CuO particle size and surface coverage. After catalyst reduction, the major absorption band lies between 2090 and 2113 cm-1 and arises from CO linearly bound to very small (1- to 5-nm) copper metal clusters. The observed frequency shifts indicate the presence of steps and terraces similar to low index Cu planes in very small particles (1- to 2-nm), and the presence of similar higher index Cu planes on larger clusters (2- to 5-nm). The absorption is described by a Freundlich isotherm with the heat of CO adsorption decreasing with coverage from 50 to 22 kJ/mol on bigger particles but more constant (27 to 22 kj/mol) on small particles. A surface copper/CO atomic ratio increasing from 5 to 12 is established at equilibrium saturation between 358 and 493 K using extinction coefficients determined in this study. In the reduced catalyst, a weakly adsorbed (ΔHa = -20 kj/mol) species assigned to CO bound to isolated Cu+ ions is also found and absorbs at 2175 cm-1. The frequency of this band does not vary with catalyst loading and is not affected by reoxidation of the catalyst in nitrous oxide.