This study presents a detailed methodology, which combines high-precision thermogravimetry and FTIR spectroscopy, aiming to establish the most accurate and reliable means of measuring the molar absorption coefficients of adsorbed species. As the integrated molar absorption coefficients of Py complexes with Bronsted and Lewis acid sites, epsilon(Py-B) and epsilon(Py-L), are determined and the validity of the Beer-Lambert-Bouguer law for IR characterisation of solid acids is demonstrated, this work is setting a benchmark for the quantitative acidity measurements in zeolites and related materials. The following values of epsilon(Py-B) have been obtained at 150 degrees C (band at similar to 1545 cm(-1)): 1.09 +/- 0.08 cm mu mol(-1) for ZSM-5; 1.12 +/- 0.16 cm mu mol(-1) for BEA; 1.29 +/- 0.04 cm mu mol(-1) for MOR and 1.54 +/- 0.15 cm( )mu mol(-1) for FAU. The value of epsilon(Py-L) (band at similar to 1455 cm(-1), which refers to different cations) measured at the same temperature is 1.71 +/- 0.1 cm( )mu mol(-1). Values of epsilon(Py-B) depend on the zeolite structure, in contrast to that for epsilon(Py-L). Clear correlations are presented between the temperature of the FTIR measurements and epsilon values for Py complexes and other species, which decrease by similar to 10% as the temperature increases by 100 degrees C. In addition, the effects of key experimental procedures, instrumentation design and sample preparation are established and quantified. (C) 2020 Elsevier Inc. All rights reserved.