An experimental investigation has been conducted of the transition in the oxidation chemistry of n-butane across the region of negative temperature coefficient from low to intermediate temperatures. The experimental results indicated a region of negative temperature coefficient between approximately 640 and 695 K and a shift in the nature of the reaction intermediates and products across this region. On the basis of these experimental results and some recent kinetic modeling results for the reaction R + O-2 reversible arrow RO(2), a new mechanism is presented for n-butane to describe the observed phenomena. At low temperatures the major reaction path of butylperoxy is isomerization followed by O-2 addition, further isomerization, and decomposition to mainly carbonyls and OH radicals. At intermediate temperatures, the major reaction of butylperoxy is isomerization followed by decomposition to butenes and HO2 and to epoxides and OH. The mechanism is consistent with these and other experimental results and predicts the negative temperature coefficient and the shift in product distribution with temperature.