A comparative FTIR spectroscopic study of CO adsorption on Cu 2 O and on silica-supported Cu 2 O microcrystals has been carried out in order to investigate the relations between the IR spectrum of adsorbed CO and the morphology of the particles (as obtained by SEM and TEM analyses). Modelling of the crystal morphology was also performed by means of atomistic simulations, and the results were compared with the electron micrographs. On both samples, CO adsorption occurs on Cu(I) ions located on the predominant and unpolar (111) faces. The simultaneous presence in the IR spectra of adsorbed CO of two components at 2158 and 2162cm - 1 (weak and fully reversible) and at 2127 and 2132cm - 1 (strong and quite irreversible), which shift downward of about Δ -8-10cm - 1 and Δ -12-14cm - 1 , respectively, as a consequence of the progressive building up of lateral interactions within the bidimensional phase, is explained in terms of two alternative models. The first model explains the presence of the two IR bands in terms of CO adsorption on the two different Cu(I) sites (mono and bicoordinated) present on the (111) face of Cu 2 O. Following the second model, the observed IR doublet is on the contrary associated with the formation of Cu(I)(CO) 2 species on monocoordinated Cu(I) sites. On the basis of the calculated electric field at the two surface sites of the relaxed and unrelaxed (111) faces, the second model seems to be more appropriate.