Alkoxy radicals (RO) are central intermediates in the atmospheric oxidation of hydrocarbons. Numerous reaction pathways are available, shown in Figure 5: isomerization, dissociation, and hydrogen-atom abstraction by oxygen. None of the three pathways are radical-termination steps; isomerization and dissociation form alkyl radicals, whereas reaction with O2 propagates the cycle, forming HO2. In the case of β-hydroxy alkoxy radicals, formed in alkene oxidation, decomposition leads to the formation of α-hydroxy alkyl radicals, which react with O2 to form HO2; this reaction is far faster than RO + O2 reactions. This branching is complicated further by the fact that RO is formed chemically activated, so there is additional competition between collisional stabilization and chemical reaction, dependent on the exothermicity of the parent reaction (RO2 + NO versus RO2 + RO2). Therefore the branching ratio of the RO reactions, which play a major role in the rate of HOX cycling, may exhibit complex dependence on pressure, temperature, and structure of the parent hydrocarbon.