The role of organic matter in heterogeneous atmospheric processes is of emerging significance. Recently the presence of films composed of a diverse organic fraction has been confirmed on impervious urban surfaces. These films can potentially sequester and react with atmospheric constituents affecting the fate of semi-volatile organic species. Organic coatings have also been confirmed on aerosols, whose composition affects their chemical and optical properties. A low-pressure effusive (Knudsen) cell reactor was designed and constructed to study uptake of organic gases with organic film proxies (oleic acid and squalene). Uptake probabilities, found to be an indicator of mass accommodation coefficient, were on the order of high 10-3 for naphthalene and chlorinated aromatics and less than 10-4 for p-xylene, alpha-pinene and acetic acid at ambient temperature. Uptake qualitatively corresponded to the octanol-air partition coefficient of gases. Preliminary results estimate a film-air partition coefficient for naphthalene in oleic acid of roughly 1.2x104.