The kinetics of the reactions of PO with O-2 and PO2 with O-3 were studied at temperatures ranging from similar to 190 to 340 K, using a pulsed laser photolysis-laser induced fluorescence technique. For the reaction of PO + O-2, there is evidence of both a two- and three-body exit channel, producing PO2 + O and PO3, respectively. Potential energy surfaces of both the PO + O-2 and PO2 + O-3 systems were calculated using electronic structure theory and combined with RRKM calculations to explain the observed pressure and temperature dependences. For PO + O-2, at pressures typical of a planetary upper atmosphere where meteoric ablation of P will occur, the reaction is effectively pressure independent with a yield of PO2 + O of >99%; the rate coefficient can be expressed by log(10) (k, 120-500 K, cm(3) molecule(-1) s(-1)) = -13.915 + 2.470 log(10) (T) - 0.5020(log(10)(T))(2), with an uncertainty of +/- 10% over the experimental temperature range (191-339 K). With increasing pressure, the yield of PO3 increases, reaching similar to 90% at a pressure of 1 atm and T = 300 K. For PO2 + O-3, k(188-339 K) = 3.7 x 10(-11) exp((-1131/T)) cm(3) molecule(-1) s(-1), with an uncertainty of +/- 26% over the stated temperature range. Laser-induced fluorescence spectra of PO over the wavelength range 245-248 nm were collected and simulated using PGOPHER to obtain new spectroscopic constants for the ground and nu = 1 vibrational levels of the X-2 Pi and A(2)Sigma(+) states of PO.