The influence of plasticizers based on carboxylic acid esters on the Fourier transform infrared (FTIR) spectroscopic determination of biodiesel contamination [fatty acid methyl esters (FAMEs)] in aviation turbine fuel was investigated. It has been found that jet fuel can become contaminated with plasticizers through the use of handling equipment based on elastomers. FTIR spectroscopic investigations show that the extent of detection of false-positive ASTM D7797 FAME levels depends upon the type of plasticizer. While aromatic carboxylic acid esters based on phthalates have no influence, aliphatic carboxylic acid esters may cause inaccurate results.