The transport of water and pentane vapor in Langmuir-Blodgett (LB) films of arachidic acid is investigated A quartz cr),stal microbalance is used to monitor the mass increases of the films upon exposure to the vapor as a function of the pressure. By examining films of different thicknesses, it is shown that it is possible to measure both surface adsorption and solubility. Diffusivity is calculated from the rate of uptake. Water and pentane have Fickian diffusion coefficients of 4 x 10(-12) cm(2) . s(-1) and 1 x 10(-12)cm(2) . s(-1), respectively. The effect of both aging and annealing of the LB films on the vapor uptake is studied and it is shown that the uptake is sensitive to the state of the film.