Fourier transform infrared (FTIR) spectroscopy is frequently used to characterize properties of catalytic sites, identify reaction intermediates, and probe interactions between adsorbates and surfaces, making it a versatile tool for elucidating reaction mechanisms in heterogeneous catalysis. While this technique is typically regarded as qualitative or semiquantitative in nature, quantitative results can be obtained by determining integrated molar extinction coefficients (IMECs), or the amount of signal per mole of adsorbed or bulk species. IMECs for vibrational modes corresponding to pyridine adsorbed onto Brensted and Lewis acid sites have been determined in the vapor phase; however, the effect of solvents on catalytic sites cannot be captured. Liquid-phase IMECs enable quantification of adsorbates in the presence of solvents, allowing quantitative evaluations of the effect of solvents on catalysts. A method is outlined for determining IMECs of pyridine adsorbed on zeolites in the liquid phase in an attenuated total reflection (ATR) configuration. The method can also be applied to substrates other than pyridine. The IMECs for pyridine adsorbed onto Brensted and Lewis acid sites in water, acetonitrile, and ethanol are determined. The IMEC values are shown to be dependent on solvent choice, but independent of zeolite framework. (C) 2017 Elsevier Inc. All rights reserved.