Atmospheric lifetimes of methane and methyl chloroform are largely determined by the rates of their reactions with hydroxyl radical. The relative lifetimes for this loss path are inversely proportional to the ratio of the corresponding rate coefficients. In this work the relative rate constants were measured in a slow-flow, temperature controlled photochemical reactor, and were based on rates of disappearance of the parent compounds as measured by FTIR spectroscopy. The temperature range was 277--356 K. Hydroxyl radicals were generated by 254 nm photolysis of O3 in the presence of water vapor. The preferred Arrhenius expression for the results is k(CH3CCl3)/k(CH4) +0.62 exp(291/T), corresponding to a value of 1.65 at 298 K and 1.77 at 277 K. The respective uncertainties are 5% and 7%. ¿ American Geophysical Union 1992