Laser-induced reactions of O-2 to yield ozone (O-3) were investigated to estimate the quantum yield of primary odd-oxygen species production from photoabsorption by O-2 as a function of excitation laser wavelength from 232 to 255 nm. The experiments were carried out at 35 degreesC in pressurized O-2 (2.0 MPa) and O-2/CO2 mixtures (9.6 MPa). The initial slope of O-3 formation versus irradiation time was used to obtain the quantum yield of the primary odd-oxygen species, minimizing possible catalytic O-3 production initiated by the subsequent photolysis of the product O-3. The quantum yield of the primary odd-oxygen species was shown to be almost 2 in pressurized O-2 at wavelengths shorter than 242 nm, i.e., the dissociation threshold of O-2. It was less than 2 in the O-2/CO2 mixture and seemed to have a tendency to increase slightly in the shorter-wavelength region. At wavelengths between 242 and 252 mn, the quantum yield decreased monotonically with increasing laser wavelength both in O-2 and in O-2/CO2 mixtures. It became almost 0 over the wavelength of 252 nm. These findings could not be explained by the contribution of the thermal energy of O-2 in the photodissociation process alone. Although thermal dissociation of O-2(A, A', c) is not ruled out on the basis of the present experiments alone, the most likely mechanism is the thermal reaction of O-2(A, A', c) to produce O + O-3, taking into account the temperature dependence experiments of Shi and Barker.