Identification and determination of diethylene glycol (DEG) in glycerin-based products was successfully achieved using FT-IR spectroscopy. Studied samples included 0.5% to 20% by mass DEGspiked into cough syrup, two paracetamol syrup formulations, and two food supplements. The characteristic DEGwavenumbers at 881 cm-1 and 1083 cm-1 were used for its quantitative determination in the studied samples. A very good accuracy in determining the DEG fraction was achieved with a mean error% of ±2.02% to ±7.69% upon using the corrected absorbance at 881 cm-1. The corrected absorbance at 1083 cm-1 band was used in the case of paracetamol formulations and resulted in a mean error% ranging from ±2.50% to ±10.28%. The values of limit of detection of the current method ranged from 0.051% to 0.068% DEG for all studied samples.