The radical-chain thermal decomposition of neopentyl chloride was studied in the temperature range of 410–496 °C and over the pressure range of 22 to 340 mm. A small surface effect was noted after prolonged conditioning of the vessel and in a vessel of high surface/volume ratio. The reaction is of three-halves order and the rate coefficient is expressible by k3/2 (11/2 mole−1/2 s−1) = 1013.55 ± 0.67 × e−56300 ± 2100/RT. The experimental facts are shown to be consistent with a mechanism involving chlorine atoms as the principal propagating radicals, with a first-order initiation step and a termination step involving the combination of methyl and chloromethyl radicals. The relative concentrations of the various radicals, calculated from known and estimated kinetic parameters, have been shown to be dependent on the hydrogen chloride produced from the concurrent unimolecular decomposition of neopentyl chloride reported in Part I. 1,1-Dimethylcyclopropane, found as a reaction product, is believed to be formed directly from neopentyl chloride by a radical-chain mechanism.