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 YOH band by about 21 cm(-1) and by slight changes in ozone vibrational frequencies. Chemisorption of O-3 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 v(3), v(1) + v(3), and v(2) vibrational modes, respectively, of the surface ozonide O-3(-) ions. Experiments with O-18 substitution supported this assignment. Effect of CO preadsorption showed ozone chemisorption to occur on the same coordinate-unsaturated O2- surface sites that accounted for the formation of the "carbonite" CO22- ion from CO adsorbed on highly activated CaO.