Ozone adsorption on CaO pretreated at different temperatures was studied by FTIR spectroscopy at 77-300 K. Besides physisorption effects, a weak complex was observed formed by ozone molecules with the OH surface groups, which have pronounced basic properties, as shown by a downward shift of the v_OH band by about 21 cm^-1 and by slight changes in ozone vibrational frequencies. Chemisorption of O₃ occurred on CaO activated at 973 K as evidenced by the appearance of a band at 812 cm^-1 accompanied by two weaker peaks at 1859 and 625 cm^-1 attributed to the v₃, v₁ + v₃, and v₂ vibrational modes, respectively, of the surface ozonide O₃^- ions. Experiments with ¹⁸O substitution supported this assignment. Effect of CO preadsorption showed ozone chemisorption to occur on the same coordinate-unsaturated O^2- surface sites that accounted for the formation of the "carbonite" CO₂^2- ion from CO adsorbed on highly activated CaO.