Surface properties of high surface area ceria samples, either in the reduced or unreduced state, have been investigated using FT-IR spectroscopy. Upon reduction, detailed features in the background spectrum of unreduced samples, which may be assigned to surface or multiphonon modes, vanish while weak bands due to electronic transitions appear. One of these bands is assignable to Ce3+ as point defect in the core. Adsorption of probe molecules is used to characterise the Lewis acid-base strength of surface sites. Adsorption of a proton donor (pyrrole) or an electronic acceptor (CO2) is indicative of the high basicity of surface O2- ions for ceria either reduced or not. The acid strength of cerium ions is weak; its decrease upon reduction may be shown by adsorbing weak Lewis bases (CO, acetonitrile) but not by stronger ones (pyridine, dimethylether). Superoxide (O-2(-)) or peroxide (O-2(2-)) surface species are produced when O-2 is adsorbed on surface reduced defects. In the case of samples with a higher degree of reduction, the electron donor power is shown by tetracyanoethylene adsorption. Surface hydroxy and methoxy species from H-2 and methanol dissociations, respectively, are very sensitive probes in differentiating one, two and threefold co-ordinatively unsaturated cationic sites and their reduction state. The use of methoxy species allows to quantify the ceria reduction degree through the addition of known amounts of oxygen. Adsorbed formate species are also sensitive to ceria reduction state.