The first study of the kinetics of B atoms with O-2 at low and ultra-low temperatures is reported. Rate constants have been measured from 295 down to 24 K using the pulsed-laser photolysis-laser-induced fluorescence technique in a Cinetique de Reaction en Ecoulement Supersonique Uniforme apparatus. Rate constants were found to increase monotonically with decreasing temperature. The temperature dependence of the rate constant has also been calculated using a theoretical approach based on the adiabatic capture centrifugal sudden approximation method. In the limit of zero temperature, the reactivities of the two spin orbit states of the B atom towards the O-2 molecule are also demonstrated to be different. (C) 2004 Elsevier B.V. All rights reserved.