The study introduces a method for facile evaluation of thicknesses of thin coating layers on substrates based on Attenuated Total Reflection Fourier Transfer Infra-Red (AIR-FM) spectroscopy. This approach enables fast and simple evaluation of average thickness of polymeric coatings up to several micrometers thick for a variety of substrates, once the refractive index and extinction coefficient of the coating and substrate materials are independently determined. The method yields average thickness over practically meaningful areas of up to several cm(2). It is useful for studying surface-modified or fouled substrates, e.g., separation membranes, and may apply to non-uniform or patchy coatings on rough substrates. Its utility was demonstrated for nano- and ultrafiltration membranes grafted with thin layers of low-fouling polymers. The method was validated by comparing ATR-FTIR thickness to the one independently measured by electron microscopy for well-defined spin-coated NF and OF membrane samples. (C) 2015 Elsevier B.V. All rights reserved.