Laser-induced reactions by a pulsed KrF excimer laser were studied using UV absorption spectroscopy in sub- and supercritical O-2/CO2 mixtures up to the pressure of 15 MPa (corresponding density, 17 mol dm(-3)). Although the 248 nm excimer laser photon energy is smaller than the energy required for dissociating O-2 ozone formation was observed in O-2/CO2 mixtures. Under the laser irradiation, O-3 concentration increased monotonically with the increase of the irradiation time and then stayed constant, which is satisfactorily expressed by the equation d[O-3]/dt = a - b[O-3]. a corresponds to O-3 formation rate and b to O-3 decomposition rate constant. The value of a increased with the increase of CO2 density up to 3 mol dm(-3) and was then kept almost constant with further increase. O-2 absorbs a photon to yield an oxygen molecule in the Herzberg III state O-2(A' (3)Delta(u)), being augmented along with the increase of CO2 density. In pure Or, the predominant pathway of O-3 formation is the reaction between excited O-2 in Herzberg states and ground state O-2 to yield O-3 and atomic oxygen. In high-density O-2/CO2 mixtures, O-3 is considered to be produced through reaction between the Herzberg states O-2 and CO2. Taking account of the quenching effect for the above reaction together with the augmentation of O-2 absorption of laser light by the high-density CO2, the behavior of a with respect to CO2 density was satisfactorily explained. The behavior of b suggested a certain inhibition of O-3 recovery in high-density CO2 after the photodecomposition of the product O-3, which was ascribed to the formation of CO3 from the O(D-1) reaction with Co-2. A certain cage effect for the O-3 photodecomposition was also suggested. No specific pressure effect was observed near the critical point.