Abstract: Apparatus for the determination of the oxidative stability of polymers exhibiting viscous or visco-elastic flow at the temperature of the determination is disclosed. The apparatus comprises a mixing head having means to subject polymer to shear, means to measure the oxygen content of gas, means to circulate the gas from the mixing means to the analyzing means and back to the mixing head and pressure control means to control the pressure of the gas at atmospheric pressure without extraneous dilution. In a preferred embodiment the pressure control means includes an elongated tube having a ratio of length to diameter of at least 800. A method for determining the oxidative stability of polymer under shear conditions is also disclosed. The apparatus and process may be used for the determination of the oxidative stability of, for example, elastomeric and thermoplastic polymers under conditions resembling the conditions under which such polymers are fabricated into articles.