Methods used for the study of the kinetics of exhaust hydrocarbon reactions are reviewed, compared and contrasted. The isothermal plug flow reactor which allows the determination of time resolved concentration histories of reactants, intermediate products, and final products is suggested as, perhaps, the most desirable and versatile system for the study of moderate temperature hydrocarbon oxidation reactions. The isothermal plug flow reactor allows the gas phase reactions to be studied with kinetically well-defined, repeatable, homogeneous reaction conditions that are essentially free of heterogeneous interference. Due to the detailed data obtainable and the controlled reaction conditions, kinetic mechanisms can be studied and evaluated. Investigations that used this technique are reviewed and examples cited to demonstrate the unique capabilities of the plug flow reactor.