A long-term (about nine months) isothermal degradation experiment of two different commercial polylactide (PLA) samples used for food packaging was carried out at a relatively low temperature (423 K). Thermooxidative degradations of the same polymers were carried out in a thermogravimetric (TG) analyser

at higher temperatures (453 ≤

T

≤ 523 K)

under isothermal heating conditions. The obtained set of experimental TG data was used to determine the apparent activation energy (

E

a

) of degradation through two isothermal kinetic methods. The results from long-term experiment evidenced considerable mass loss for both PLA samples in the investigated period

but the experimental data were not in agreement with those from the short-term degradations at higher temperatures

thus suggesting a different degradation kinetics

and

then a low reliability of the lifetime predictions for polymers in service or degradation forecasts for the end of their life based on experiments at higher temperatures.