Common physicomechanical tests comprising impact strength, the melt flow index (MFI), and the yellowness index (YI) were used to study the thermooxidative stability of polypropylene (PP) following multipass extrusion. Differential scanning calorimetry (DSC) coupled with chemiluminescence (CL) monitoring was also used to assess the stability. Three PP formulations were studied: ii) PP-1 containing 0.050% w/w of the phenolic antioxidant Irganox 1010(TM), (ii) PP-2 containing 0.028% w/w Irganox 1010, 0.056% w/w of the phosphite costabilizer Irgafos 168(TM) and 0.014% w/w of the lactone processing stabilizer HP 136(TM), and (iii) PP-3 containing 0.050% w/w Irganox 1010 and 0.100% w/w of the phosphite Ultranox 641(TM). All formulations contained 0.045% w/w of the hydrotalcite acid scavenger DHT-4A(TM). The results suggest that physicomechanical tests cannot reliably detect the small difference in the stability between PP-2 and PP-3 but can detect the larger difference between these and the less stable PP-1. The oxidative induction time (OIT) determined by CL monitoring(i.e., CL-OIT) is consistent with the OIT determined by DSC but has better reliability. The CL-OIT data suggest that PP-3 has superior oxidative stability to PP-2 in the early stages of multipass extrusion. However, PP-2 exhibits a better resistance to yellowing. A correlation between the CL-OIT data and each of the MFI and YI data was found.