In this article, the degradation mechanism of PE films containing additives with metal ions at a simulated composting temperature has been studied. The hydroperoxide concentration [POOH] in the films was traced quantitatively by using the iodometric potentiometric titration, compared with Fourier transform infrared spectrometry (FTIR). The results show that [POOH] increases during the early stage of degradation, followed by a more or less flat maximum before it starts to decrease. At the same time, similar results are obtained by the FTIR analysis. Besides, it is found that the rate laws for the carbonyl index and [POOH] increases seem more complicated, instead of an exponential-type increase in the early stage of the oxidation. When they are fitted by polynomial equations, the greater the rate of degradation, the more the number and content of the high-power members in the polynomial.