The thermal decomposition of the tert-butoxyl radical has been studied in the gas phase in the range 398–436°K at pressures of 10–60 mm using di-tert-butyl peroxide + nitric oxide mixtures. The reaction is pressure dependent and the following Arrhenius parameters for the high and low-pressure limiting rates have been determined by a Hinshelwood-Lindemann treatment logA∞(sec–1)= 14.7, E∞(kcal mole–1)= 22.8, log A0(mole–1 cm3 sec–1)= 15.7 and E0(kcal mole–1)= 13.4. The half-rate pressure at 436°K is 112 mm, and the results have been explained in terms of a model corresponding to about 23 effective oscillators in the activated state.