Commercially available poly(ethylene terephthalate) (PET) in the shape of bottles and foils has been thermally aged, which simulates atmospheric thermal aging. The aging of plastic waste has been studied in order to gain insight into the degradation of plastics in the environment, which over time turns into microplastics (MP) and to assess their impact on the environment. The samples of PET foil was 0.75 mm thick and PET bottles 0.25 mm thick have been thermally aged for a period of 14, 28 and 42 days, with a daily cycle that was as follow: 16h at 70°C, 2h at -18°C and 6h at 70°C with an increased amount of moisture. After aging the samples have been analysed with FTIR spectrometer where the characteristic signals of functional groups within the molecule of PET have been recorded, of which the most important was the carbonyl group. From that data the Carbonyl index (C.I.) was calculated, which change indicate the degradation of the polymer molecules. Also, the samples have been analysed using DSC method where the glass transition temperature (Tg) and the degree of crystallinity (X) over time have been determined. The results of this paper indicate that the effect of aging is low due to the short periods of aging time because PET polymer exhibited exceptional stability under atmospheric conditions. Although, some changes in the structure of PET molecules have been noted, which indicate the degradation of the macromolecules and the formation of molecules of lower molecular masses. From the results a notable decrease in the crystallinity is visible and the C.I. has decreased by 33% after 42 days of thermal aging.