The decomposition of dimethyl peroxide (DMP) was studied in the presence and absence of added NO, to determine rate constants k l and k z in the temperature range of 391-432OK:The results reconcile the studies by Takezaki and Takeuchi, Hanst and Calvert, and Batt and McCulloch, giving log kl(sec-1) = (15.7 f 0.5) -(37.1 f 0.9)/2.3 RT and k , = 5 X 104MM-'sec-1. The disproportionation/recombination ratio k7t,/k7, = 0.30 f 0.05 was also determined: Me0 + NO, (+ &I) % MeONOz (+ M ) M e 0 + NO, 3 CH,O + HONO When 0 2 was added to DMP mixtures containing NO,, relative rate constants klZ/k?a CH,O + 0 2 3 CHIO + HOZ were obtained over the temperature range of 396-442'K: A review of literature data produced kTa = 109.s*~.~M-1~sec-1, giving log k12(M-1.sec-1) = (8.5 f 1.5) -(4.0 =k 2.8)/2.3 RT, where most of the uncertainty is due to the limited temperature range of the experiments. *Present address: