Chemical processes are studied in ozone formation by the photodissociation of oxygen molecule excited to the Schumann-Runge band at 193 nm with use of an ArF excimer laser. The experimental results at an O-2 pressure of 100-500 Torr indicate that the ozone formation O + O-2 + O-2 --> O-3 + O-2 is very rapid, that the predissociation O-2 --> O + O is a rate-determining process. The rate constant of the predissociation O-2 --> O + O is found to be 6.30 x 10(-5) cm(2) J(-1) at 22.1 degrees C.