The reaction of allyl radicals with oxygen has been studied using argon fluoride laser flash photolysis coupled with ultraviolet absorption detection. At temperatures between 382 and 453 K it was possible to observe directly the establishment of equilibrium between allyl, oxygen and allyl peroxy (C₃H₅+ O₂⇌ C₃H₅O₂). From analysis of the allyl radical decay signals the temperature dependences of both the forward and reverse rate constants, k₁ and k_–1, were obtained. The temperature dependence of the equilibrium constant, K_p=k₁/(k_–1RT) gave a value for ΔS^⊖₂₉₈ of –(122 ± 5) J mol^–1 K^–1 and for ΔH^⊖₂₉₈ of –(76.2 ± 2.1) kJ mol^–1. The equilibrium constants were shown to be pressure-independent, although both k₁ and k_–1 were below their high-pressure limits. Above 453 K the form of the decay and the apparent value of the equilibrium constant deviate from the lower-temperature behaviour because of the onset of an additional reaction channel.