Due to massive production, inefficient waste collection, and long lives, plastics have become a source of persistent pollution. Biodegradation is explored as an environmentally friendly remediation method for removing plastics from the environment. Microbial and animal biodegradation methods have been reported in the literature for various plastics. Levels of plastic oxidation are often used as an evidence of degradation and can be measured with great sensitivity by Fourier Transform Infrared (FTIR) spec- troscopy. FTIR is highly sensitive to the creation of new CAO, C@O and OAH bonds during oxidation. However, many studies reporting the use of FTIR spectroscopy to evidence plastic oxidation confused the spectral signatures of biomass contamination (CAO and C@O from lipids, CONH from proteins, O– H from polysaccharides) with plastic oxidation. Here, based on spectra of oxidized plastic and of probable contaminants, we make recommendations for performing and analyzing FTIR measurements properly.