The kinetics of the reactions of atomic bromine with ethene, 2-methylpropane, dimethyl ether, and diethyl ether have been studied at 298 K in a 70-L photochemical reaction chamber with the relative rate method. Chemical analysis was by gas chromatography with a flame ionization detector. The effects of bromine and oxygen partial pressure and of the choice of reference reactant on the numerical values of the rate constants obtained have been evaluated. The results are analyzed to obtain limiting values of the rate constant ratio that are independent of the concentration of O-2 and do not depend on the mechanism used to represent the overall reaction. The following rate constants were obtained at 298 K by using the well-established rate constant for the reaction of Br with 2-methylpropane as the primary reference: Br + (CH3)(2)O, k(298) = 9.43 x 10(5) L mol(-1) s(-1) +/-2%; Br + (C2H5)(2)O, k(298) = 1.41 x 10(7) L mol(-1) s(-1) +/- 6%. With the rate constants for Br + (CH3)(2)O as a secondary reference value, the rate constant at 298 K for Br + (C2H5)(2)O was 1.27 x 10(7) L mol(-1) s(-1) +/- 8% calculated at a limiting high concentration Of O-2. When Br + (C2H5)(2)O was measured relative to Br + C2H4, the rate constant at 298 K, calculated for limiting high concentrations of O-2, ranged from 4.7 x 10(7) to 1.2 x 10(7) L mol(-1) s(-1) depending on the rate constant chosen for the reference reaction. The sensitivity to oxygen concentration of the rate constant for the reaction of Br with ethene depends on the concentration of Br? used as the photolytic source of atomic bromine. When the reaction of Br with an organic reactant is sensitive to the presence of oxygen, measurements at O-2 concentrations much larger than those in synthetic air may be required to obtain reliable rate constants for the initial reaction of Br with the organic reactant.