The reaction of CN with O-2 has been studied through a photolysis-probe laser experiment in a cell at a total pressure of 70 mTorr. Rotationally hot CN reagent was prepared by 193 nm photolysis of BrCN. NCO((X) over tilde (2) Pi) product in various vibronic levels was detected by laser fluorescence excitation in its (A) over tilde (2) Sigma(+)-(X) over tilde (2) Pi band system at variable delays after the photolysis laser. In order to monitor the CN collisional relaxation which is taking place simultaneously with the reaction, we have also derived the CN rotational state distribution as a function of the photolysis-probe delay from laser fluorescence excitation spectra of the CN B (2) Sigma+-X (2) Sigma(+) (0,0) band. From these observations, we deduce that rotationally hot CN reacts more slowly than thermalized CN. Moreover, reaction of the former yields NCO product with greater bending vibronic excitation. These results are compared with previous dynamical studies of this reaction, carried out with crossed beams and in cells. O 1995 American Institute of Physics.