An extensive series of: (i) oxidative induction time (OIT) measurements derived from differential scanning calorimetry experiments, and (ii) stabilizer analyses by high performance liquid chromatography were conducted on medium-density polyethylene (MDPE) stabilized with various levels of Irganox 1010 (penta-erythrityl-tetrakis-(3,5-di-tert-4-hydroxyphenyl propionate), Irgafos 168 (tris-(2,4-di-tert-butylphenyl)-phosphite), and carbon black (CB). The OIT data were analyzed using a novel interpretation method, which is described and applied. It was found that: (i) addition of CB to base-stabilized MDPE increases stability and reduces OIT variability; (ii) the optimum effective concentrations of Irganox 1010, Irgafos 168 and CB are 1690 ppm, 1580 ppm and 2.5% w/w, respectively; and (iii) interactions between CB and the base co-stabilizers may occur at high loadings of the latter but CB does not affect the extraction of the base co-stabilizers which are extracted with efficiencies of ca. 80% and 100% for Irganox 1010 and Irgafos 168, respectively.