The service life of a polyolefin product depends to large extent on the type and amount of the antioxidants added. During the manufacturing, storage and use of the product the antioxidants are depleted by physical processes and chemical degradation, and this impairs its long-term performance. The initial and in-use oxidation stability is often characterized and monitored by the measurement of the oxidative induction time (OIT), and service life predictions are based on the rate of decrease of the OIT value. To study the correlation between the OIT value and the actual antioxidant concentration, eight random arrays of high-impact polypropylene (PP) strands stabilized using six different antioxidant packages (composed of Irgafos 168, Irganox 1010 and 1330, Chimassorb 944) were immersed in hot water (80°C and 90°C) and oven aged in air (80°C) for more than two years, and the change of OIT values and antioxidant concentrations was measured. For phosphitic and phenolic stabilizers water immersion is generally a more critical aging condition than oven aging in air, while for the hindered amine stabilizers (HAS) the opposite was observed. As expected, a linear correlation between OIT values and concentrations was found for the “classical” package of phosphitic and phenolic stabilizers. In the case of Irganox 1330, the change of the OIT values during aging was considerably slower than the change of concentration. Even when zero concentration was reached according to the vanishing peak height in the chromatogram, a considerable OIT value was still measurable. This may be due to unidentified degradation products of Irganox 1330 which still act as antioxidants in the long run. Addition of HAS (Chimassorb 944) seems to enhance the initial OIT of stabilized phenolic samples and the long-term effectiveness of the phenolic stabilization under air oven aging conditions. However, the marked long-term effectiveness of the HAS itself and the slow change with time of the concentration were not detectable by OIT measurements above the PP melting point of about 170°C.