A range of formulations — consisting of high-density polyethylene, surface-coated calcium carbonate, masterbatched carbon black and a stabiliser package — were compared in terms of their thermal stability through the use of several techniques and measures. Notably, calcium carbonate loadings of up to 60 wt% were investigated. The formulations were designed to be I- and D-optimal, based on a quadratic Scheffé polynomial model. Novel and established measures for the thermal stability of the materials were compared, based on data generated using techniques for the determination of oxidative induction time and for studies by time-sweep oscillatory rheometry. For ease of comparison, all tests were performed in air. Fourier-transform infrared spectroscopy was performed on the materials subjected to time-sweep oscillatory rheometry, to ascertain the locality and modes of degradation. Variable effects were found, depending on the technique used. Broadly, strong primary effects were found: positive in the cases of the carbon black and stabiliser pack, negative in the case of calcium carbonate. The latter, however, was offset by the highly synergistic interaction of calcium carbonate with the stabiliser pack. Time-sweep oscillatory rheometry was shown to be a promising method, with it yielding high-quality, comparable results when interpreted using a novel measure.