The gas-phase reactions of O₂, HO_x and NO_x and of selected volatile organic compounds, such as alkanes, alkenes, and aromatic hydrocarbons, in the troposphere is discussed. In recent years much progress has been made in elucidating reactions of alkoxy radicals, the mechanisms of gas-phase reactions of O₃ with alkenes, and the mechanisms of formation of products of the ^oOH radical-initiated reactions of aromatic hydrocarbons. There are still areas of uncertainty which impact on the ability to model precisely the formation of tropospheric ozone: rate constants and mechanisms of reaction of organic peroxy radicals RO₂^o with NO, NO₃^o, HO₂^o and other RO₂^o radicals; organic nitrate formation by reactions of RO₂^o with NO; reaction rates of alkoxy radicals in decomposition, isomerization, and reaction with O₂; detailed mechanisms of reactions of O₃ with alkenes; mechanisms and products of reactions of OH-aromatic adducts with O₂ and NO_x.