An isothermal oxidation apparatus (IOA) has been modified to include an oxygen sensor so that both oxidation and deposition data can be measured for jet fuels that are thermally stressed. This modification is useful for comparing the relative oxidation rates of jet fuels and fuels blended with additives. Antioxidants can be evaluated in this apparatus by comparing the delay in the onset of oxidation at a given temperature in a fast oxidizing fuel. Other additive types, such as dispersants, can be evaluated by their ability to reduce deposition.