We have studied the adsorption of organic molecules from the vapor phase onto bare gold, n-octadecanethiol-modified, and 11-mercapto-1-undecanol-modified gold surfaces using a controlled atmosphere chamber with an ellipsometer and a quartz crystal microbalance for simultaneous in-situ determination of adsorbate thickness and mass. Our experimental data are fit by the BET adsorption isotherm, and results from these fits place limits on the ways in which the adsorbates can interact with the selected substrates. Ellipsometric data, in combination with the mass adsorption data, demonstrate that n-octadecanethiol self-assembled monolayers on gold experience penetration by adsorbates, where the 11-mercapto-1-undecanol-modified surfaces and the gold surface do not. We compare these findings to other literature reports that bear on the permeability of the alkanethiol/gold interface.