To reveal the antifreeze protection mechanism of an aqueous solution of ethylene glycol (EG) at a molecular level, concentration-dependent infrared spectra are analyzed with an aid of chemometrics. The principal component analysis (PCA) of the spectra reveals that the spectral variation is explained by the quantity changes not only of 'bulky water' and the 'bulky EG,' but also of a 'water/EG complex.' After a spectral decomposition using the alternative least squares (ALS) analysis, the spectrum of the complex reveals that the EG molecule keeps the gauche conformation, and the terminal hydroxyl groups are hydrogen bonded by water molecules, which is a key to understand the antifreeze effect. In addition, the complex is found to comprises an EG molecule with four water molecules. Since the quantity of the complex attains the maximum at an EG concentration of 60 wt%, at which the freezing point becomes lowest, the complex is concluded to be a key hydrated species for the antifreeze effect. The generation process of the water/EG complex is also studied by using the time-resolved IR spectroscopy, which consistently confirms the spectral discussion made above. (C) 2016 Elsevier B.V. All rights reserved.