The adsorption of CO on partially reduced CeO₂[CeO₂(573-H) and CeO₂(673-H), CeO₂ reduced in H₂ at 573 and 673 K for 1 h, respectively] has been studied by Fourier-transform infrared spectroscopy (F.t.i.r.). The observation of formate species (771, 1329, 1369, 1558, 1587, 2852 and 2945 cm^–1) formed by the reaction of CO with the surface of CeO₂(673-H) at room temperature is reported. Two weak bands at 2796 and 2706 cm^–1, tentatively attributed to formyl species, were also detected at room temperature when the bands of the formate species no longer appeared to grow. No band was observed when CO was dosed on CeO₂(573-H) or CeO₂(673-H₂O)(CeO₂ hydrated at 673 K for 1 h) at room temperature and 373 K. Two sharp bands at 3685 and 3643 cm^–1, due to isolated hydroxyl groups, and a broad band centred at 3421 cm^–1 were generated on CeO₂(673-H) through H₂ reduction. Among these only the OH groups giving the 3685 cm^–1 band manifest activity to CO at room temperature. A mechanism involving a formyl intermediate was proposed to explain the formation of formate species on partially reduced CeO₂. The partial reduction of CeO₂ causes pronounced inhibition of the formation of carbonate-like species from CO at room temperature. This is consistent with the conclusions reported in Part 1 of this work.