The Quartz Crystal Microbalance (QCM) system is utilized to investigate the relationship between mass uptake and associated swelling for Langmuir-Blodgett (LB) organic thin films obtained from pyrene end-capped polystyrene (PS). The study was carried out using three different molecular weights of polymeric chains. The changes in resonance frequency associated with mass changes can be attributed to the swelling behavior of polymeric thin films during vapor absorption. This swelling is due to the capturing of organic vapor molecules in the sensor environment. To quantify real-time QCM data for swelling, early-time Fick's law of diffusion was adopted to fit the results, and a good linear relationship was observed between the mass uptake and square root of the swelling time. The diffusion coefficients for swelling were thus obtained from the slopes of the fitting curves and was found to be correlation with the amount of organic vapor content in the cell. It was also observed that diffusion of the organic vapor into higher molecular weight polystyrene thin films are much faster than low molecular weight ones in sensor applications. Diffusion coefficients were found to be 0.2-3.0 x 10(-16), 5.0-13 x 10(-16), and 1.0-1.6 x 10(-15) cm(2)/s for PS1, PS2, and PS3 LB thin films, respectively. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 2414-2422, 2012