The gas-phase free radical displacement reactionhas been studied in the temperature range of 240-290°C and at 14OOC with the thermal decomposition of azomethane (AM) and di-tert -butylperoxide (DTBP), respectively, as methyl radical sources. The reaction products of the CD3 radicals were analyzed by mass spectrometry. Assuming negligible isotope effects, Arrhenius parameters for the elementary radical addition reactionwere derived: loglo kl(cm3/mol sec) = (10.5 f 0.4) -(11,500 f 1100)/4.5762'The data are discussed with respect to the back reaction and general features of elementary addition reactions.