Sunflower oil (SFO) is preferably chosen for frying because it gets soaked less and is very light. Nevertheless, there is a possibility that SFO is adulterated with thermally deteriorated oil re-used such as the leftover oil after the frying process of meat with breadcrumbs (TDO A) and potatoes (TDO B). It may be very harmful to our health and the environment, requiring a fast and reliable screening test for such adulterants. In this study, Fourier transform infrared spectroscopy based on the attenuated total reflectance (ATR-FITR) sampling technique was studied for the direct determination of TDOA and TDO B as adulterants in SFO-TDO binary mixtures. Our proposed quantification methods were based on the simple linear regression analysis of IR spectral intensity (peak height or peak area) at the wavenumber 1746 cm − 1 on adulterant concentration 0 ÷ 10% (v/v) (R2 > 0.990). They proved to be accurate (% recovery in the range of 98.0 ÷ 101.1%), precise (RSD < 2.7%) and technically simple (total analysis time of 5–10 min).