Relative rate studies were carried out to determine the temperature dependent rate constant ratio k(1)/k(2a): CH3O center dot + O-2 -> HCHO + HO2 center dot and CH3O center dot + NO2 (+M) -> CH3ONO2 (+M) over the temperature range 250-333 K in an environmental chamber at 700 Torr using Fourier transform infrared detection. Absolute rate constants k(2) were determined using laser flash photolysis/laser-induced fluorescence under the same conditions. The analogous experiments were carried out for the reactions of the perdeuterated methoxy radical (CD3O center dot). Absolute rate constants k(2) were in excellent agreement with the recommendations of the JPL Data Evaluation panel. The combined data (i.e., k(1)/k(2) and k(2)) allow the determination of k(1) as 1.31(-0.5)(+0.9) x 10(-14) exp[-(663 +/- 144)/T] cm(3) s(-1), corresponding to 1.4 x 10(-15) cm(3) s(-1) at 298 K. The rate constant at 298 K is in excellent agreement with previous work, but the observed temperature dependence is less than was previously reported. The deuterium isotope effect, k(H)/k(D), can be expressed in the Arrhenius form as k(1)/k(3) = (1.7(-0.4)(+0.5)) exp((306 +/- 70)/T). The deuterium isotope effect does not appear to be greatly influenced by tunneling, which is consistent with a previous theoretical work by Hu and Dibble. (Hu, H.; Dibble, T. S., J. Phys. Chem. A 2013, 117, 14230-14242.)