The UV-vis and FTIR spectra of CO, adsorbed at the surface of alpha-Cr2O3 microcrystals, were preliminarily studied to gain information on the nature of the active sites (coordination number and local structure) present on the most abundant (01(1) over bar 2), ((2) over bar 116), and (11(2) over bar 0) faces. On these faces two main families of Cr3+ (5-fold and, to a lesser extent, 4-fold coordinated) are present, which form, with carbon monoxide, sigma-pi 1:1 adducts, with predominant sigma character. The molecular adsorption of ethene involves the 5-fold Cr3+ sites of the predominant (01(1) over bar 2) faces, giving a C-2v coordination complex, where ethene interacts with the electron-withdrawing Cr3+ centers, through a sigma-bond, perpendicular to the plane of the olefinic system. On high index faces, the ethene molecule oligomerizes, the polymerization activity of reduced samples being definitely larger than that of stoichiometric samples. The presence of a small fraction of reduced centers on higher index faces and on edges is responsible for the increased polymerization rate and for the formation of longer polymeric chains. The problem of the interaction of the hydrocarbon chains, formed at the catalytic centers, with the Cr3+ sites of the predominant (but not catalytically relevant) faces is also considered, by studying the interaction of a model hydrocarbon (n-heptane) with the clean surface. The effect of CO redosing on the alpha-Cr2O3 surfaces, precovered by hydrocarbon chains, is studied and the remarkable difference in the frequency and intensity of the CO peaks is discussed.