Molecular adsorption on 2D membranes plays a key role in surface-mediated permeation offering selectivity benefits for chemical separation. As many vaporous compounds are demonstrated to pass through 2D membranes faster than ordinary gases, it is important to determine their surface coverage on flat substrates under realistic conditions. Here we present a viable reference system to quantify polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS) measurements with organic vapors. Microscopic deposits of poly(1-trimethylsilyl-1-propyne) (PTMSP) onto metallic films are introduced as a matrix that soaks up liquid substances and accommodates them during the spectral acquisition. The corresponding molar uptake is obtained with a microbalance and converted into an areal density allowing for direct coverage-intensity relationships. We probe room-temperature physisorption of methanol, ethanol, and n-propanol on supported carbon nanomembranes and correlate the number of adsorbates with the mass transfer rates in free-standing layers. This work opens a new dimension for adsorption controlled permeation (ACP) studies.