The rate constant for abstraction of hydrogen from isobutane by methyl radicals has been re-examined making use of new measurements at temperatures above 660 K. Most of the data over the temperature range 300-855 K can be accommodated by the following values for the rate constants for abstraction from primary and tertiary hydrogen atoms:This activation energy for abstraction of the tertiary hydrogen atom in isobutane is essentially the same a s expressed in a previous evaluation, which considered data only below 500 K, but the magnitude of the rate constant is about 0.25 log units greater.Recently, the rate constant for hydrogen abstraction from ethylene by methyl radicals i l l was measured relative to the corresponding abstraction reaction, With the reactant isobutane [2], on the other hand, again using the evaluated data for this reference reaction [41, extrapolated to the temperature range