The thermal reaction of sulfenates (RS−OR), yielding their corresponding sulfoxides (RS(O)R), was studied experimentally. The first step of the reaction was found to be the formation a radical pair by homolytic cleavage of the carbon−oxygen bond of the sulfenate. The two transient radicals formed then recombine to form the carbon−sulfur bond of the sulfoxide. The thermolysis of cinnamyl-4-nitrobenzenesulfenate has a positive entropy of activation (ΔS⧧ = 6.4 ± 2.0 eu in toluene), characteristic of a dissociative pathway. A normal secondary kinetic isotope effect (kH/kD = 1.19 ± 0.04) was also measured with this substrate. Finally, a trapping experiment allowed the isolation and characterization of a product coming from the coupling of the cinnamyl radical and TEMPO. These studies confirm a mechanism that was proposed earlier based on computational studies. The experimentally determined bond dissociation energy of the carbon−oxygen bond of ∼28 kcal·mol-1 is in good agreement with the computed value of ∼26 k...